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, 2016
Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 5,946,144 $ 6,159,590
Unrealized Gross Gains 100,277 16,282
Unrealized Gross Losses (4,178) (40,414)
Fair Value 6,042,243 6,135,458
Federal agencies: Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,335,953 5,532,501
Unrealized Gross Gains 89,932 14,637
Unrealized Gross Losses (4,178) (37,504)
Fair Value 5,421,707 5,509,634
Federal agencies, Other agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 603,442 620,052
Unrealized Gross Gains 10,329 1,645
Unrealized Gross Losses 0 (2,786)
Fair Value 613,771 618,911
Total U.S. Treasury, Federal agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,939,395 6,152,553
Unrealized Gross Gains 100,261 16,282
Unrealized Gross Losses (4,178) (40,290)
Fair Value 6,035,478 6,128,545
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,749 7,037
Unrealized Gross Gains 16 0
Unrealized Gross Losses 0 (124)
Fair Value $ 6,765 $ 6,913