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Investment Securities - Summary of Components and Unrealized Losses on Held-to-Maturity Securities (Details)
$ in Thousands
Dec. 31, 2019
USD ($)
Investment
Dec. 31, 2018
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 13,842,946 $ 15,487,442
Unrealized Gains 289,763 17,040
Unrealized Losses (17,437) (316,246)
Fair Value 14,115,272 15,188,236
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,218,980 3,851,352
Unrealized Losses, Less than 12 months (5,727) (56,636)
Fair Value of Investments, 12 months or longer 1,368,956 8,092,085
Unrealized Losses, 12 months or longer (11,710) (259,610)
Fair Value of Investments 2,587,936 11,943,437
Unrealized Losses $ (17,437) $ (316,246)
Number of held-to-maturity investments with unrealized loss | Investment 266 1,244
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 143 695
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 518,728 $ 640,990
Unrealized Gains 6,640 2,148
Unrealized Losses (668) (4,850)
Fair Value 524,700 638,288
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 125,304 291,432
Unrealized Losses, Less than 12 months (668) (2,915)
Fair Value of Investments, 12 months or longer 0 66,624
Unrealized Losses, 12 months or longer 0 (1,935)
Fair Value of Investments 125,304 358,056
Unrealized Losses (668) (4,850)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,992,009 8,103,638
Unrealized Gains 142,209 5,011
Unrealized Losses (2,066) (157,767)
Fair Value 7,132,152 7,950,882
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 132,042 2,493,156
Unrealized Losses, Less than 12 months (420) (34,956)
Fair Value of Investments, 12 months or longer 181,585 3,972,690
Unrealized Losses, 12 months or longer (1,646) (122,811)
Fair Value of Investments 313,627 6,465,846
Unrealized Losses (2,066) (157,767)
Residential mortgage-backed securities | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,608,032 2,183,204
Unrealized Gains 592 0
Unrealized Losses (8,502) (62,272)
Fair Value 1,600,122 2,120,932
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 350,868 16,952
Unrealized Losses, Less than 12 months (1,131) (109)
Fair Value of Investments, 12 months or longer 872,527 2,103,980
Unrealized Losses, 12 months or longer (7,371) (62,163)
Fair Value of Investments 1,223,395 2,120,932
Unrealized Losses (8,502) (62,272)
Residential mortgage-backed securities | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 178,611 214,483
Unrealized Gains 94 608
Unrealized Losses (259) (14)
Fair Value 178,446 215,077
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 143,265 3,364
Unrealized Losses, Less than 12 months (256) (1)
Fair Value of Investments, 12 months or longer 4,615 8,101
Unrealized Losses, 12 months or longer (3) (13)
Fair Value of Investments 147,880 11,465
Unrealized Losses (259) (14)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,759,615 2,769,706
Unrealized Gains 56,914 6,969
Unrealized Losses (4,508) (64,374)
Fair Value 2,812,021 2,712,301
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 307,087 177,697
Unrealized Losses, Less than 12 months (1,818) (1,580)
Fair Value of Investments, 12 months or longer 310,229 1,600,277
Unrealized Losses, 12 months or longer (2,690) (62,794)
Fair Value of Investments 617,316 1,777,974
Unrealized Losses (4,508) (64,374)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,785,951 1,575,421
Unrealized Gains 83,314 2,304
Unrealized Losses (1,434) (26,969)
Fair Value 1,867,831 1,550,756
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 160,414 868,751
Unrealized Losses, Less than 12 months (1,434) (17,075)
Fair Value of Investments, 12 months or longer 0 340,413
Unrealized Losses, 12 months or longer 0 (9,894)
Fair Value of Investments 160,414 1,209,164
Unrealized Losses $ (1,434) $ (26,969)