Investment Securities - Components of Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 15,898,263	$ 12,663,455
Unrealized Gains	3,572	12,923
Unrealized Losses	(407,840)	(128,098)
Fair Value	15,493,995	12,548,280
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	713,809	659,979
Unrealized Gains	0	3,167
Unrealized Losses	(13,818)	(1,601)
Fair Value	699,991	661,545
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,588,483	6,304,969
Unrealized Gains	1,591	4,854
Unrealized Losses	(210,148)	(43,528)
Fair Value	8,379,926	6,266,295
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,494,834	2,829,979
Unrealized Gains	0	23
Unrealized Losses	(91,919)	(54,372)
Fair Value	2,402,915	2,775,630
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	233,285	255,782
Unrealized Gains	691	733
Unrealized Losses	(31)	(34)
Fair Value	233,945	256,481
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,335,971	1,868,985
Unrealized Gains	10	694
Unrealized Losses	(62,982)	(25,563)
Fair Value	2,272,999	1,844,116
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,531,881	743,761
Unrealized Gains	1,280	3,452
Unrealized Losses	(28,942)	(3,000)
Fair Value	$ 1,504,219	$ 744,213