Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Total	$ 9,593,366
One Year or Less	2,593,259
After One Year to Five Years	4,280,052
After Five Years to Ten Years	392,721
After Ten Years	2,327,334
U.S. treasury securities
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Total	5,733,070
One Year or Less	2,037,522
After One Year to Five Years	3,349,925
After Five Years to Ten Years	345,623
After Ten Years	0
U.S. agency debentures
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Total	1,485,864
One Year or Less	555,737
After One Year to Five Years	930,127
After Five Years to Ten Years	0
After Ten Years	0
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Total	2,038,134
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	47,098
After Ten Years	1,991,036
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Total	336,298
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	$ 336,298
Available-for-sale Securities | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Available-for-sale Securities | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years