Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,776,896	$ 7,862,311
Unrealized Gains	20,062	12,082
Unrealized Losses	(41,864)	(84,350)
Carrying Value	6,755,094	7,790,043
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,767,777	4,762,182
Unrealized Gains	19,425	11,638
Unrealized Losses	(19,792)	(35,562)
Carrying Value	3,767,410	4,738,258
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,066,279	1,090,426
Unrealized Gains	394	61
Unrealized Losses	(2,960)	(6,370)
Carrying Value	1,063,713	1,084,117
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,689	5,815
Unrealized Gains	0	0
Unrealized Losses	(4)	(3)
Carrying Value	5,685	5,812
Residential mortgage-backed securities | Fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,858,979	1,922,618
Unrealized Gains	0	0
Unrealized Losses	(19,096)	(42,400)
Carrying Value	1,839,883	1,880,218
Residential mortgage-backed securities | Variable rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,172	81,270
Unrealized Gains	243	383
Unrealized Losses	(12)	(15)
Carrying Value	$ 78,403	$ 81,638