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Investment Securities - Summary of Components and Unrealized Losses on Held-to-Maturity Securities (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 15,487,442 $ 12,663,455
Unrealized Gains 17,040 12,923
Unrealized Losses (316,246) (128,098)
Fair Value 15,188,236 12,548,280
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 3,851,352 6,710,931
Unrealized Losses, Less than 12 months (56,636) (62,993)
Fair Value of Investments, 12 months or longer 8,092,085 2,966,895
Unrealized Losses, 12 months or longer (259,610) (65,105)
Fair Value of Investments 11,943,437 9,677,826
Unrealized Losses $ (316,246) $ (128,098)
Number of held-to-maturity investments with unrealized loss | Investment 1,244 753
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 695 237
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 640,990 $ 659,979
Unrealized Gains 2,148 3,167
Unrealized Losses (4,850) (1,601)
Fair Value 638,288 661,545
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 291,432 104,688
Unrealized Losses, Less than 12 months (2,915) (1,601)
Fair Value of Investments, 12 months or longer 66,624 0
Unrealized Losses, 12 months or longer (1,935) 0
Fair Value of Investments 358,056 104,688
Unrealized Losses (4,850) (1,601)
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 8,103,638 6,304,969
Unrealized Gains 5,011 4,854
Unrealized Losses (157,767) (43,528)
Fair Value 7,950,882 6,266,295
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 2,493,156 4,270,377
Unrealized Losses, Less than 12 months (34,956) (34,092)
Fair Value of Investments, 12 months or longer 3,972,690 408,913
Unrealized Losses, 12 months or longer (122,811) (9,436)
Fair Value of Investments 6,465,846 4,679,290
Unrealized Losses (157,767) (43,528)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,183,204 2,829,979
Unrealized Gains 0 23
Unrealized Losses (62,272) (54,372)
Fair Value 2,120,932 2,775,630
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 16,952 1,011,709
Unrealized Losses, Less than 12 months (109) (13,631)
Fair Value of Investments, 12 months or longer 2,103,980 1,741,614
Unrealized Losses, 12 months or longer (62,163) (40,741)
Fair Value of Investments 2,120,932 2,753,323
Unrealized Losses (62,272) (54,372)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 214,483 255,782
Unrealized Gains 608 733
Unrealized Losses (14) (34)
Fair Value 215,077 256,481
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 3,364 0
Unrealized Losses, Less than 12 months (1) 0
Fair Value of Investments, 12 months or longer 8,101 9,812
Unrealized Losses, 12 months or longer (13) (34)
Fair Value of Investments 11,465 9,812
Unrealized Losses (14) (34)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,769,706 1,868,985
Unrealized Gains 6,969 694
Unrealized Losses (64,374) (25,563)
Fair Value 2,712,301 1,844,116
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 177,697 979,361
Unrealized Losses, Less than 12 months (1,580) (11,566)
Fair Value of Investments, 12 months or longer 1,600,277 773,712
Unrealized Losses, 12 months or longer (62,794) (13,997)
Fair Value of Investments 1,777,974 1,753,073
Unrealized Losses (64,374) (25,563)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,575,421 743,761
Unrealized Gains 2,304 3,452
Unrealized Losses (26,969) (3,000)
Fair Value 1,550,756 744,213
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 868,751 344,796
Unrealized Losses, Less than 12 months (17,075) (2,103)
Fair Value of Investments, 12 months or longer 340,413 32,844
Unrealized Losses, 12 months or longer (9,894) (897)
Fair Value of Investments 1,209,164 377,640
Unrealized Losses $ (26,969) $ (3,000)