Quarterly report pursuant to Section 13 or 15(d)

HELD-TO-MATURITY SECURITIES - Contractual Maturities (Details)

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HELD-TO-MATURITY SECURITIES - Contractual Maturities (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost $ 7,533,517 $ 7,806,939
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 7,502,102 7,787,268
Federal agencies: Mortgage-backed securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity Securities, Under 1 year 0 0
Held-to-maturity Securities, 1-5 years 0 0
Held-to-maturity Securities, 6-10 years 59,710 41,261
Held-to-maturity Securities, Over 10 years 6,887,466 7,157,083
Amortized Cost 6,947,176 7,198,344
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 0 0
Fair Value, 6-10 years 59,420 40,791
Fair Value, Over 10 years 6,857,540 7,139,943
Fair Value 6,916,960 7,180,734
Federal agencies, Other agencies    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity Securities, Under 1 year 0 0
Held-to-maturity Securities, 1-5 years 0 0
Held-to-maturity Securities, 6-10 years 382,617 398,341
Held-to-maturity Securities, Over 10 years 197,714 204,083
Amortized Cost 580,331 602,424
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 0 0
Fair Value, 6-10 years 383,290 399,452
Fair Value, Over 10 years 195,990 201,180
Fair Value 579,280 600,632
Total U.S. Treasury, Federal agency securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost 7,527,507 7,800,768
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 7,496,240 7,781,366
Municipal securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity Securities, Under 1 year 0 0
Held-to-maturity Securities, 1-5 years 0 0
Held-to-maturity Securities, 6-10 years 0 0
Held-to-maturity Securities, Over 10 years 6,010 6,171
Amortized Cost 6,010 6,171
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 0 0
Fair Value, 6-10 years 0 0
Fair Value, Over 10 years 5,862 5,902
Fair Value $ 5,862 $ 5,902