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
Sep. 30, 2016
Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 5,301,387 $ 6,159,590
Unrealized Gross Gains 91,045 16,282
Unrealized Gross Losses (3,339) (40,414)
Fair Value 5,389,093 6,135,458
Federal agencies: Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 4,720,093 5,532,501
Unrealized Gross Gains 78,826 14,637
Unrealized Gross Losses (3,338) (37,504)
Fair Value 4,795,581 5,509,634
Federal agencies, Other agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 574,920 620,052
Unrealized Gross Gains 12,219 1,645
Unrealized Gross Losses 0 (2,786)
Fair Value 587,139 618,911
Total U.S. Treasury, Federal agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,295,013 6,152,553
Unrealized Gross Gains 91,045 16,282
Unrealized Gross Losses (3,338) (40,290)
Fair Value 5,382,720 6,128,545
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,374 7,037
Unrealized Gross Gains 0 0
Unrealized Gross Losses (1) (124)
Fair Value $ 6,373 $ 6,913