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Investment Securities - Summary Held to Maturity Securities (Details)
$ in Thousands
Dec. 31, 2016
USD ($)
Investment
Dec. 31, 2015
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 8,426,998 $ 8,790,963
Unrealized Gains 16,968 16,694
Unrealized Losses (67,828) (49,035)
Fair Value 8,376,138 8,758,622
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 5,943,190 6,302,783
Unrealized Losses, Less than 12 months (60,189) (44,391)
Fair Value of Investments, 12 months or longer 295,657 238,350
Unrealized Losses, 12 months or longer (7,639) (4,644)
Fair Value of Investments 6,238,847 6,541,133
Unrealized Losses $ (67,828) $ (49,035)
Number of held-to-maturity investments with unrealized loss | Investment 462 384
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 85 58
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 622,445 $ 545,473
Unrealized Gains 7,840 8,876
Unrealized Losses (1,198) 0
Fair Value 629,087 554,349
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 118,721  
Unrealized Losses, Less than 12 months (1,198)  
Fair Value of Investments, 12 months or longer 0  
Unrealized Losses, 12 months or longer 0  
Fair Value of Investments 118,721  
Unrealized Losses (1,198)  
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,896,179 2,366,627
Unrealized Gains 6,919 546
Unrealized Losses (24,526) (11,698)
Fair Value 2,878,572 2,355,475
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,801,861 2,121,258
Unrealized Losses, Less than 12 months (23,558) (10,860)
Fair Value of Investments, 12 months or longer 21,917 22,507
Unrealized Losses, 12 months or longer (968) (838)
Fair Value of Investments 1,823,778 2,143,765
Unrealized Losses (24,526) (11,698)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 3,362,598 4,225,781
Unrealized Gains 788 3,054
Unrealized Losses (31,274) (32,999)
Fair Value 3,332,112 4,195,836
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 2,729,889 3,153,483
Unrealized Losses, Less than 12 months (25,723) (30,230)
Fair Value of Investments, 12 months or longer 228,220 150,058
Unrealized Losses, 12 months or longer (5,551) (2,769)
Fair Value of Investments 2,958,109 3,303,541
Unrealized Losses (31,274) (32,999)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 312,665 370,779
Unrealized Gains 176 758
Unrealized Losses (1,339) (33)
Fair Value 311,502 371,504
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 251,012 170,350
Unrealized Losses, Less than 12 months (1,339) (33)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 251,012 170,350
Unrealized Losses (1,339) (33)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,151,363 1,214,716
Unrealized Gains 1,237 3,405
Unrealized Losses (7,638) (3,475)
Fair Value 1,144,962 1,214,646
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 999,440 823,414
Unrealized Losses, Less than 12 months (7,494) (2,994)
Fair Value of Investments, 12 months or longer 14,934 40,276
Unrealized Losses, 12 months or longer (144) (481)
Fair Value of Investments 1,014,374 863,690
Unrealized Losses (7,638) (3,475)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 81,748 67,587
Unrealized Gains 8 55
Unrealized Losses (1,853) (830)
Fair Value 79,903 66,812
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 42,267 34,278
Unrealized Losses, Less than 12 months (877) (274)
Fair Value of Investments, 12 months or longer 30,586 25,509
Unrealized Losses, 12 months or longer (976) (556)
Fair Value of Investments 72,853 59,787
Unrealized Losses $ (1,853) $ (830)