Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUES OF ASSETS AND LIABILITIES - Level 3 Roll Forward (Details)

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FAIR VALUES OF ASSETS AND LIABILITIES - Level 3 Roll Forward (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs $ (1) $ (3) $ (1) $ (3) $ (1) $ (2)
Available-for-sale securities 13,727   13,727   14,869  
Assets Level 3 Roll Forward:            
Transfers out of Level 3 (1)     (26) (4)    
Total gains/losses for the period:            
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings     25 9    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)     0 0    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases     0 0    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales     0 0    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements     3 0    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Change in Unrealized Gain (Loss)     (1) (9)    
MSRs            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3     0 0    
Assets Level 3 Roll Forward:            
Opening balance 11 13 11 14 14  
Transfers out of Level 3 (1) 0 0        
Total gains/losses for the period:            
Included in earnings 0 (1) 0 (2)    
Included in OCI 0 0 0 0    
Purchases/originations 0 0 0 0    
Sales 0 0 0 0    
Repayments 0 0 0 0    
Settlements 0 0 0 0    
Closing balance 11 12 11 12 11  
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date 0 (1) 0 (2)    
Derivative instruments            
Assets Level 3 Roll Forward:            
Opening balance 1 3        
Transfers out of Level 3 (1) (12) (1)        
Total gains/losses for the period:            
Included in earnings (9) (1)        
Included in OCI 0 0        
Purchases/originations 0 0        
Sales 0 0        
Repayments 0 0 0 0    
Settlements (3) 0        
Closing balance 1 3 1 3    
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date (3) (1)        
Municipal securities            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3     0 0    
Assets Level 3 Roll Forward:            
Opening balance 3,178 2,872 3,167 2,798 2,798  
Transfers out of Level 3 (1) 0 0        
Total gains/losses for the period:            
Included in earnings (1) (1) (3) (4)    
Included in OCI 0 34 (37) (1)    
Purchases/originations 260 167 539 414    
Sales 0 0 0 0    
Repayments 0 0 0 0    
Settlements (160) (46) (389) (249)    
Closing balance 3,277 2,958 3,277 2,958 3,167  
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date 0 0 0 0    
Asset-backed securities            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3     0 0    
Assets Level 3 Roll Forward:            
Opening balance   43 24 76 76  
Transfers out of Level 3 (1)   0        
Total gains/losses for the period:            
Included in earnings   (2) (2) (5)    
Included in OCI   (5) (11) (13)    
Purchases/originations   0 0 0    
Sales   (22) (33) (59)    
Repayments   0 0 0    
Settlements   0 0 (1)    
Closing balance 0 24 0 24 24  
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date   0 0 (4)    
Automobile            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3     0 0    
Assets Level 3 Roll Forward:            
Opening balance 34 44 38 48 48  
Transfers out of Level 3 (1) 0 0        
Total gains/losses for the period:            
Included in earnings 0 0 0 (1)    
Included in OCI 0 0 0 0    
Purchases/originations 0 0 0 0    
Sales 0 0 0 0    
Repayments (2) (4) (6) (9)    
Settlements 0 0 0 0    
Closing balance 32 40 32 40 38  
Change in unrealized gains or losses for the period included in earnings for assets held at end of the reporting date 0 $ 0 0 $ 0    
Commercial Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 1,703   1,703   2,487  
Residential Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 620   620   1,367  
Fair Value, Measurements, Recurring [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 13,727   13,727   14,869  
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 1,703   1,703   2,487  
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 620   620   1,367  
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 10,445   10,445   11,673  
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities 1,703   1,703   2,487  
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities [Member]            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Available-for-sale securities $ 620   $ 620   $ 1,367