Annual report pursuant to Section 13 and 15(d)

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

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HELD-TO-MATURITY SECURITIES - Contractual Maturities (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Held-to-maturity Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]    
Amortized Cost $ 7,806,939 $ 6,159,590
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]    
Held-to-maturity Securities, Fair Value 7,787,268 6,135,458
Federal agencies: Mortgage-backed    
Held-to-maturity Securities, Debt Maturities, Amortized Cost, Rolling Maturity [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 41,261 25,909
Held-to-maturity Securities, Over 10 years 7,157,083 5,506,592
Amortized Cost 7,198,344 5,532,501
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 0 0
Fair Value, 6-10 years 40,791 25,227
Fair Value, Over 10 Years 7,139,943 5,484,407
Held-to-maturity Securities, Fair Value 7,180,734 5,509,634
Federal agencies: Other agencies    
Held-to-maturity Securities, Debt Maturities, Amortized Cost, Rolling Maturity [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 398,341 283,960
Held-to-maturity Securities, Over 10 years 204,083 336,092
Amortized Cost 602,424 620,052
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]    
Fair Value, Under 1 year 0 0
Fair Value, 1-5 years 0 0
Fair Value, 6-10 years 399,452 284,907
Fair Value, Over 10 Years 201,180 334,004
Held-to-maturity Securities, Fair Value 600,632 618,911
Total U.S. Government backed agencies    
Held-to-maturity Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]    
Amortized Cost 7,800,768 6,152,553
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]    
Held-to-maturity Securities, Fair Value 7,781,366 6,128,545
Municipal securities    
Held-to-maturity Securities, Debt Maturities, Amortized Cost, Rolling Maturity [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 6,171 7,037
Amortized Cost 6,171 7,037
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling 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 5,902 6,913
Held-to-maturity Securities, Fair Value $ 5,902 $ 6,913