Quarterly report pursuant to Section 13 or 15(d)

AVAILABLE-FOR-SALE AND OTHER SECURITIES - Contractual Maturities (Details)

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AVAILABLE-FOR-SALE AND OTHER SECURITIES - Contractual Maturities (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost $ 15,611,683 $ 15,776,855
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 15,453,061 15,562,837
U.S. Treasury    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 11,256 4,978
Amortized Cost, 1-5 years 0 502
Amortized Cost, 6-10 years 0 0
Amortized Cost, Over 10 years 0 0
Amortized Cost 11,256 5,480
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 11,260 4,988
Fair Value, 1-5 years 0 509
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 11,260 5,497
Federal agencies: Mortgage-backed securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 0 0
Amortized Cost, 1-5 years 32,749 46,591
Amortized Cost, 6-10 years 257,032 173,941
Amortized Cost, Over 10 years 10,496,277 10,630,929
Amortized Cost 10,786,058 10,851,461
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 32,515 46,762
Fair Value, 6-10 years 255,488 176,404
Fair Value, Over 10 years 10,351,747 10,450,176
Available-for-sale and other securities 10,639,750 10,673,342
Federal agencies, Other agencies    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 4,201 4,302
Amortized Cost, 1-5 years 8,892 5,092
Amortized Cost, 6-10 years 82,692 63,618
Amortized Cost, Over 10 years 0 0
Amortized Cost 95,785 73,012
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 4,223 4,367
Fair Value, 1-5 years 9,034 5,247
Fair Value, 6-10 years 83,194 63,928
Fair Value, Over 10 years 0 0
Available-for-sale and other securities 96,451 73,542
Total U.S. Treasury, Federal agency securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 10,893,099 10,929,953
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 10,747,461 10,752,381
Municipal securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 163,747 169,636
Amortized Cost, 1-5 years 905,872 933,893
Amortized Cost, 6-10 years 1,656,860 1,463,459
Amortized Cost, Over 10 years 703,350 693,440
Amortized Cost 3,429,829 3,260,428
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 160,032 166,887
Fair Value, 1-5 years 905,075 933,903
Fair Value, 6-10 years 1,655,384 1,464,583
Fair Value, Over 10 years 705,618 684,684
Available-for-sale and other securities 3,426,109 3,250,057
Asset-backed securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 0 0
Amortized Cost, 1-5 years 80,003 80,700
Amortized Cost, 6-10 years 162,079 223,352
Amortized Cost, Over 10 years 326,724 520,072
Amortized Cost 568,806 824,124
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 80,330 80,560
Fair Value, 6-10 years 163,439 224,565
Fair Value, Over 10 years 311,422 488,356
Available-for-sale and other securities 555,191 793,481
Corporate debt    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 3,143 43,223
Amortized Cost, 1-5 years 66,878 78,430
Amortized Cost, 6-10 years 38,471 32,523
Amortized Cost, Over 10 years 13,211 40,361
Amortized Cost 121,703 194,537
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 3,157 43,603
Fair Value, 1-5 years 68,450 80,196
Fair Value, 6-10 years 39,902 32,865
Fair Value, Over 10 years 14,120 42,019
Available-for-sale and other securities 125,629 198,683
Other securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost, Under 1 year 3,150 1,650
Amortized Cost, 1-5 years 800 2,302
Amortized Cost, 6-10 years 0 0
Amortized Cost, Over 10 years 0 10
Amortized Cost 598,246 567,813
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 3,144 1,650
Fair Value, 1-5 years 791 2,283
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 0 10
Available-for-sale and other securities 598,671 568,235
Nonmarketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 583,019 547,704
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 583,019 547,704
Mutual funds    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 10,416 15,286
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities 10,416 15,286
Marketable equity securities    
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]    
Amortized Cost 861 861
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Available-for-sale and other securities $ 1,301 $ 1,302