Annual report pursuant to Section 13 and 15(d)

HELD-TO-MATURITY SECURITIES - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, & Fair Value (Details)

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HELD-TO-MATURITY SECURITIES - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, & Fair Value (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Held-to-maturity Securities [Abstract]    
Amortized Cost $ 7,806,939 $ 6,159,590
Unrealized Gross Gains 22,573 16,282
Unrealized Gross Losses (42,244) (40,414)
Fair Value 7,787,268 6,135,458
Federal agencies: Mortgage-backed    
Held-to-maturity Securities [Abstract]    
Amortized Cost 7,198,344 5,532,501
Unrealized Gross Gains 20,883 14,637
Unrealized Gross Losses (38,493) (37,504)
Fair Value 7,180,734 5,509,634
Federal agencies: Other agencies    
Held-to-maturity Securities [Abstract]    
Amortized Cost 602,424 620,052
Unrealized Gross Gains 1,690 1,645
Unrealized Gross Losses (3,482) (2,786)
Fair Value 600,632 618,911
Total U.S. Government backed agencies    
Held-to-maturity Securities [Abstract]    
Amortized Cost 7,800,768 6,152,553
Unrealized Gross Gains 22,573 16,282
Unrealized Gross Losses (41,975) (40,290)
Fair Value 7,781,366 6,128,545
Municipal securities    
Held-to-maturity Securities [Abstract]    
Amortized Cost 6,171 7,037
Unrealized Gross Gains 0 0
Unrealized Gross Losses (269) (124)
Fair Value $ 5,902 $ 6,913