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, 2016
Dec. 31, 2015
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity securities $ 5,658,565 $ 6,159,590
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 5,786,224 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 43,441 25,909
Held-to-maturity Securities, Over 10 years 5,015,636 5,506,592
Held-to-maturity securities 5,059,077 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 44,543 25,227
Fair Value, Over 10 years 5,126,798 5,484,407
Fair Value 5,171,341 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 309,750 283,960
Held-to-maturity Securities, Over 10 years 283,113 336,092
Held-to-maturity securities 592,863 620,052
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 317,300 284,907
Fair Value, Over 10 years 290,830 334,004
Fair Value 608,130 618,911
Total U.S. Treasury, Federal agency securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held-to-maturity securities 5,651,940 6,152,553
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Fair Value 5,779,471 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 6,625 7,037
Held-to-maturity securities 6,625 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 6,753 6,913
Fair Value $ 6,753 $ 6,913