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
Mar. 31, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 7,533,517 $ 7,806,939
Unrealized Gross Gains 13,420 22,573
Unrealized Gross Losses (44,835) (42,244)
Fair Value 7,502,102 7,787,268
Federal agencies: Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,947,176 7,198,344
Unrealized Gross Gains 11,534 20,883
Unrealized Gross Losses (41,750) (38,493)
Fair Value 6,916,960 7,180,734
Federal agencies, Other agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 580,331 602,424
Unrealized Gross Gains 1,886 1,690
Unrealized Gross Losses (2,937) (3,482)
Fair Value 579,280 600,632
Total U.S. Treasury, Federal agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 7,527,507 7,800,768
Unrealized Gross Gains 13,420 22,573
Unrealized Gross Losses (44,687) (41,975)
Fair Value 7,496,240 7,781,366
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,010 6,171
Unrealized Gross Gains 0 0
Unrealized Gross Losses (148) (269)
Fair Value $ 5,862 $ 5,902