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
Jun. 30, 2017
Dec. 31, 2016
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost $ 8,279,577 $ 7,806,939
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 8,259,402 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 70,527 41,261
Held-to-maturity Securities, Over 10 years 7,634,775 7,157,083
Amortized Cost 7,705,302 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 70,355 40,791
Fair Value, Over 10 years 7,616,513 7,139,943
Fair Value 7,686,868 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 376,837 398,341
Held-to-maturity Securities, Over 10 years 191,592 204,083
Amortized Cost 568,429 602,424
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 376,750 399,452
Fair Value, Over 10 years 190,149 201,180
Fair Value 566,899 600,632
Total U.S. Treasury, Federal agency securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost 8,273,731 7,800,768
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 8,253,767 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 5,846 6,171
Amortized Cost 5,846 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 5,635 5,902
Fair Value $ 5,635 $ 5,902