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, 2016
Dec. 31, 2015
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity securities $ 5,946,144 $ 6,159,590
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 6,042,243 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 45,210 25,909
Held-to-maturity Securities, Over 10 years 5,290,743 5,506,592
Held-to-maturity securities 5,335,953 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 45,848 25,227
Fair Value, Over 10 years 5,375,859 5,484,407
Fair Value 5,421,707 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 274,849 283,960
Held-to-maturity Securities, Over 10 years 328,593 336,092
Held-to-maturity securities 603,442 620,052
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 279,716 284,907
Fair Value, Over 10 years 334,055 334,004
Fair Value 613,771 618,911
Total U.S. Treasury, Federal agency securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity securities 5,939,395 6,152,553
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 6,035,478 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 6,749 7,037
Held-to-maturity securities 6,749 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 6,765 6,913
Fair Value $ 6,765 $ 6,913