Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 15,899,726	$ 12,663,455
Fair Value	15,372,238	12,548,280
One Year or Less, Amortized Cost	8,376
One Year or Less, Fair Value	8,373
After One Year to Five Years, Amortized Cost	358,915
After One Year to Five Years, Fair Value	350,493
After Five Years to Ten Years, Amortized Cost	2,126,938
After Five Years to Ten Years, Fair Value	2,039,827
After Ten Years, Amortized Cost	13,405,497
After Ten Years, Fair Value	12,973,545
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	641,134	659,979
Fair Value	622,737	661,545
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	104,550
After Five Years to Ten Years, Amortized Cost	536,584
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	8,333,088	6,304,969
Fair Value	8,056,342	6,266,295
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	179,847
After Five Years to Ten Years, Amortized Cost	859,233
After Ten Years, Amortized Cost	7,294,008
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
Fair Value	2,230,506	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	474,668
After Ten Years, Amortized Cost	1,854,397
After Ten Years, Fair Value	1,778,078
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
Fair Value	224,009	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	223,374
After Ten Years, Fair Value	224,009
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,795,952	1,868,985
Fair Value	2,717,264	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	2,795,952
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,577,113	743,761
Fair Value	1,521,380	$ 744,213
One Year or Less, Amortized Cost	8,376
After One Year to Five Years, Amortized Cost	74,518
After Five Years to Ten Years, Amortized Cost	256,453
After Ten Years, Amortized Cost	$ 1,237,766
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
Held-to-maturity Securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
After One Year to Five Years, Fair Value	$ 102,498
After Five Years to Ten Years, Fair Value	520,239
Held-to-maturity Securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
After One Year to Five Years, Fair Value	174,664
After Five Years to Ten Years, Fair Value	821,883
After Ten Years, Fair Value	7,059,795
Held-to-maturity Securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
After Five Years to Ten Years, Fair Value	452,428
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
After Ten Years, Fair Value	2,717,264
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
One Year or Less, Fair Value	8,373
After One Year to Five Years, Fair Value	73,331
After Five Years to Ten Years, Fair Value	245,277
After Ten Years, Fair Value	$ 1,194,399