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Investment Securities - Summary Held to Maturity Securities (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 12,663,455 $ 8,426,998
Unrealized Gains 12,923 16,968
Unrealized Losses (128,098) (67,828)
Fair Value 12,548,280 8,376,138
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 6,710,931 5,943,190
Unrealized Losses, Less than 12 months (62,993) (60,189)
Fair Value of Investments, 12 months or longer 2,966,895 295,657
Unrealized Losses, 12 months or longer (65,105) (7,639)
Fair Value of Investments 9,677,826 6,238,847
Unrealized Losses $ (128,098) $ (67,828)
Number of held-to-maturity investments with unrealized loss | Investment 753 462
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 237 85
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 659,979 $ 622,445
Unrealized Gains 3,167 7,840
Unrealized Losses (1,601) (1,198)
Fair Value 661,545 629,087
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 104,688 118,721
Unrealized Losses, Less than 12 months (1,601) (1,198)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 104,688 118,721
Unrealized Losses (1,601) (1,198)
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 6,304,969 2,896,179
Unrealized Gains 4,854 6,919
Unrealized Losses (43,528) (24,526)
Fair Value 6,266,295 2,878,572
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 4,270,377 1,801,861
Unrealized Losses, Less than 12 months (34,092) (23,558)
Fair Value of Investments, 12 months or longer 408,913 21,917
Unrealized Losses, 12 months or longer (9,436) (968)
Fair Value of Investments 4,679,290 1,823,778
Unrealized Losses (43,528) (24,526)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,829,979 3,362,598
Unrealized Gains 23 788
Unrealized Losses (54,372) (31,274)
Fair Value 2,775,630 3,332,112
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,011,709 2,729,889
Unrealized Losses, Less than 12 months (13,631) (25,723)
Fair Value of Investments, 12 months or longer 1,741,614 228,220
Unrealized Losses, 12 months or longer (40,741) (5,551)
Fair Value of Investments 2,753,323 2,958,109
Unrealized Losses (54,372) (31,274)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 255,782 312,665
Unrealized Gains 733 176
Unrealized Losses (34) (1,339)
Fair Value 256,481 311,502
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 0 251,012
Unrealized Losses, Less than 12 months 0 (1,339)
Fair Value of Investments, 12 months or longer 9,812 0
Unrealized Losses, 12 months or longer (34) 0
Fair Value of Investments 9,812 251,012
Unrealized Losses (34) (1,339)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,868,985 1,151,363
Unrealized Gains 694 1,237
Unrealized Losses (25,563) (7,638)
Fair Value 1,844,116 1,144,962
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 979,361 999,440
Unrealized Losses, Less than 12 months (11,566) (7,494)
Fair Value of Investments, 12 months or longer 773,712 14,934
Unrealized Losses, 12 months or longer (13,997) (144)
Fair Value of Investments 1,753,073 1,014,374
Unrealized Losses (25,563) (7,638)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 743,761 81,748
Unrealized Gains 3,452 8
Unrealized Losses (3,000) (1,853)
Fair Value 744,213 79,903
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 344,796 42,267
Unrealized Losses, Less than 12 months (2,103) (877)
Fair Value of Investments, 12 months or longer 32,844 30,586
Unrealized Losses, 12 months or longer (897) (976)
Fair Value of Investments 377,640 72,853
Unrealized Losses $ (3,000) $ (1,853)