Investment Securities - Summary Held to Maturity Securities (Details) $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment	Dec. 31, 2013 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Transfers from available-for-sale securities to held-to-maturity, carrying value	$ 0	$ 5,418,572	$ 0
Amortized Cost	8,790,963	7,421,042
Unrealized Gains	16,694	16,314
Unrealized Losses	(49,035)	(21,700)
Fair Value	8,758,622	7,415,656
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	6,302,783	3,438,894
Unrealized Losses, Less than 12 months	(44,391)	(16,520)
Fair Value of Investments, 12 months or longer	238,350	783,558
Unrealized Losses, 12 months or longer	(4,644)	(5,180)
Fair Value of Investments	6,541,133	4,222,452
Unrealized Losses	$ (49,035)	$ (21,700)
Number of held-to-maturity investments with unrealized loss \| Investment	384	292
Number of held-to-maturity investments in continuous loss more than 12 months \| Investment	58	26
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 545,473	$ 405,899
Unrealized Gains	8,876	4,589
Unrealized Losses	0	(38)
Fair Value	554,349	410,450
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months		48,335
Unrealized Losses, Less than 12 months		(38)
Fair Value of Investments, 12 months or longer		0
Unrealized Losses, 12 months or longer		0
Fair Value of Investments		48,335
Unrealized Losses		(38)
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,366,627	2,799,923
Unrealized Gains	546	5,789
Unrealized Losses	(11,698)	(2,320)
Fair Value	2,355,475	2,803,392
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	2,121,258	999,230
Unrealized Losses, Less than 12 months	(10,860)	(2,320)
Fair Value of Investments, 12 months or longer	22,507	0
Unrealized Losses, 12 months or longer	(838)	0
Fair Value of Investments	2,143,765	999,230
Unrealized Losses	(11,698)	(2,320)
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,225,781	3,185,109
Unrealized Gains	3,054	4,521
Unrealized Losses	(32,999)	(14,885)
Fair Value	4,195,836	3,174,745
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	3,153,483	1,682,348
Unrealized Losses, Less than 12 months	(30,230)	(9,705)
Fair Value of Investments, 12 months or longer	150,058	783,558
Unrealized Losses, 12 months or longer	(2,769)	(5,180)
Fair Value of Investments	3,303,541	2,465,906
Unrealized Losses	(32,999)	(14,885)
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	370,779	131,580
Unrealized Gains	758	371
Unrealized Losses	(33)	0
Fair Value	371,504	131,951
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	170,350
Unrealized Losses, Less than 12 months	(33)
Fair Value of Investments, 12 months or longer	0
Unrealized Losses, 12 months or longer	0
Fair Value of Investments	170,350
Unrealized Losses	(33)
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,214,716	814,589
Unrealized Gains	3,405	1,026
Unrealized Losses	(3,475)	(3,800)
Fair Value	1,214,646	811,815
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	823,414	629,840
Unrealized Losses, Less than 12 months	(2,994)	(3,800)
Fair Value of Investments, 12 months or longer	40,276	0
Unrealized Losses, 12 months or longer	(481)	0
Fair Value of Investments	863,690	629,840
Unrealized Losses	(3,475)	(3,800)
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	67,587	83,942
Unrealized Gains	55	18
Unrealized Losses	(830)	(657)
Fair Value	66,812	83,303
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	34,278	79,141
Unrealized Losses, Less than 12 months	(274)	(657)
Fair Value of Investments, 12 months or longer	25,509	0
Unrealized Losses, 12 months or longer	(556)	0
Fair Value of Investments	59,787	79,141
Unrealized Losses	$ (830)	$ (657)

Available-For-Sale Securities, Transfers To Held-To-Maturity, Carrying Value

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