Quarterly report pursuant to Section 13 or 15(d)

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

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HELD-TO-MATURITY SECURITIES - Amortized Cost, Gross Unrealized Gains and Losses, & Fair Value (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 8,279,577 $ 7,806,939
Unrealized Gross Gains 19,324 22,573
Unrealized Gross Losses (39,499) (42,244)
Fair Value 8,259,402 7,787,268
Federal agencies: Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 7,705,302 7,198,344
Unrealized Gross Gains 17,950 20,883
Unrealized Gross Losses (36,384) (38,493)
Fair Value 7,686,868 7,180,734
Federal agencies, Other agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 568,429 602,424
Unrealized Gross Gains 1,374 1,690
Unrealized Gross Losses (2,904) (3,482)
Fair Value 566,899 600,632
Total U.S. Treasury, Federal agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 8,273,731 7,800,768
Unrealized Gross Gains 19,324 22,573
Unrealized Gross Losses (39,288) (41,975)
Fair Value 8,253,767 7,781,366
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,846 6,171
Unrealized Gross Gains 0 0
Unrealized Gross Losses (211) (269)
Fair Value $ 5,635 $ 5,902