Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 15,898,263	$ 12,663,455
Fair Value	15,493,995	12,548,280
One Year or Less, Amortized Cost	7,964
One Year or Less, Fair Value	7,942
After One Year to Five Years, Amortized Cost	370,580
After One Year to Five Years, Fair Value	363,341
After Five Years to Ten Years, Amortized Cost	1,935,948
After Five Years to Ten Years, Fair Value	1,873,346
After Ten Years, Amortized Cost	13,583,771
After Ten Years, Fair Value	13,249,366
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	713,809	659,979
Fair Value	699,991	661,545
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	122,040
After One Year to Five Years, Fair Value	120,081
After Five Years to Ten Years, Amortized Cost	591,769
After Five Years to Ten Years, Fair Value	579,910
After Ten Years, Amortized Cost	0
After Ten Years, Fair Value	0
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,588,483	6,304,969
Fair Value	8,379,926	6,266,295
One Year or Less, Amortized Cost	707
One Year or Less, Fair Value	706
After One Year to Five Years, Amortized Cost	172,594
After One Year to Five Years, Fair Value	168,271
After Five Years to Ten Years, Amortized Cost	672,564
After Five Years to Ten Years, Fair Value	648,623
After Ten Years, Amortized Cost	7,742,618
After Ten Years, Fair Value	7,562,326
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
Fair Value	2,402,915	2,775,630
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	412,530
After Five Years to Ten Years, Fair Value	394,779
After Ten Years, Amortized Cost	2,082,304
After Ten Years, Fair Value	2,008,136
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
Fair Value	233,945	256,481
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	0
After Five Years to Ten Years, Fair Value	0
After Ten Years, Amortized Cost	233,285
After Ten Years, Fair Value	233,945
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,335,971	1,868,985
Fair Value	2,272,999	1,844,116
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	0
After Five Years to Ten Years, Fair Value	0
After Ten Years, Amortized Cost	2,335,971
After Ten Years, Fair Value	2,272,999
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,531,881	743,761
Fair Value	1,504,219	$ 744,213
One Year or Less, Amortized Cost	7,257
One Year or Less, Fair Value	7,236
After One Year to Five Years, Amortized Cost	75,946
After One Year to Five Years, Fair Value	74,989
After Five Years to Ten Years, Amortized Cost	259,085
After Five Years to Ten Years, Fair Value	250,034
After Ten Years, Amortized Cost	1,189,593
After Ten Years, Fair Value	$ 1,171,960
Held-to-maturity Securities | Lower Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Held-to-maturity Securities | Upper Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years