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, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value $ 6,285,058 $ 3,460,695
Amortized Cost 8,688,399 7,806,939
Unrealized Gross Gains 16,565 22,573
Unrealized Gross Losses (49,159) (42,244)
Fair Value 8,655,805 7,787,268
Federal agencies: Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value 6,031,533 3,041,586
Amortized Cost 8,136,625 7,198,344
Unrealized Gross Gains 14,868 20,883
Unrealized Gross Losses (47,238) (38,493)
Fair Value 8,104,255 7,180,734
Federal agencies, Other agencies    
Schedule of Held-to-maturity Securities [Line Items]    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value 248,109 413,207
Amortized Cost 546,208 602,424
Unrealized Gross Gains 1,697 1,690
Unrealized Gross Losses (1,771) (3,482)
Fair Value 546,134 600,632
Total U.S. Treasury, Federal agency securities    
Schedule of Held-to-maturity Securities [Line Items]    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value 6,279,642 3,454,793
Amortized Cost 8,682,833 7,800,768
Unrealized Gross Gains 16,565 22,573
Unrealized Gross Losses (49,009) (41,975)
Fair Value 8,650,389 7,781,366
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value 5,416 5,902
Amortized Cost 5,566 6,171
Unrealized Gross Gains 0 0
Unrealized Gross Losses (150) (269)
Fair Value $ 5,416 $ 5,902