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
Sep. 30, 2016
Dec. 31, 2015
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost $ 5,301,387 $ 6,159,590
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 5,389,093 6,135,458
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 42,431 25,909
Held-to-maturity Securities, Over 10 years 4,677,662 5,506,592
Amortized Cost 4,720,093 5,532,501
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 43,449 25,227
Fair Value, Over 10 years 4,752,132 5,484,407
Fair Value 4,795,581 5,509,634
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 311,223 283,960
Held-to-maturity Securities, Over 10 years 263,697 336,092
Amortized Cost 574,920 620,052
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 317,298 284,907
Fair Value, Over 10 years 269,841 334,004
Fair Value 587,139 618,911
Total U.S. Treasury, Federal agency securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Amortized Cost 5,295,013 6,152,553
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 5,382,720 6,128,545
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 6,374 7,037
Amortized Cost 6,374 7,037
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 6,373 6,913
Fair Value $ 6,373 $ 6,913