Investment Securities - Components of Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 15,899,726	$ 12,663,455
Unrealized Gains	915	12,923
Unrealized Losses	(528,403)	(128,098)
Fair Value	15,372,238	12,548,280
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	641,134	659,979
Unrealized Gains	85	3,167
Unrealized Losses	(18,482)	(1,601)
Fair Value	622,737	661,545
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,333,088	6,304,969
Unrealized Gains	2	4,854
Unrealized Losses	(276,748)	(43,528)
Fair Value	8,056,342	6,266,295
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,329,065	2,829,979
Unrealized Gains	0	23
Unrealized Losses	(98,559)	(54,372)
Fair Value	2,230,506	2,775,630
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	223,374	255,782
Unrealized Gains	667	733
Unrealized Losses	(32)	(34)
Fair Value	224,009	256,481
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,795,952	1,868,985
Unrealized Gains	0	694
Unrealized Losses	(78,688)	(25,563)
Fair Value	2,717,264	1,844,116
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,577,113	743,761
Unrealized Gains	161	3,452
Unrealized Losses	(55,894)	(3,000)
Fair Value	$ 1,521,380	$ 744,213