Investment Securities - Summary of Remaining Contractual Principal Maturities for Available-for-Sale Securities (Detail) - Available-for-sale Securities $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Investments Classified By Contractual Maturity Date [Line Items]
Total	$ 7,790,043
One Year or Less	2,431,083
After One Year to Five Years	2,948,663
After Five Years to Ten Years	464,471
After Ten Years	1,945,826
U.S. treasury securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	4,738,258
One Year or Less	1,765,333
After One Year to Five Years	2,524,484
After Five Years to Ten Years	448,441
After Ten Years	0
U.S. agency debentures
Investments Classified By Contractual Maturity Date [Line Items]
Total	1,084,117
One Year or Less	665,750
After One Year to Five Years	418,367
After Five Years to Ten Years	0
After Ten Years	0
Foreign government debt securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	5,812
One Year or Less	0
After One Year to Five Years	5,812
After Five Years to Ten Years	0
After Ten Years	0
Residential mortgage-backed securities | Fixed rate
Investments Classified By Contractual Maturity Date [Line Items]
Total	1,880,218
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	16,030
After Ten Years	1,864,188
Residential mortgage-backed securities | Variable rate
Investments Classified By Contractual Maturity Date [Line Items]
Total	81,638
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	$ 81,638
Lower Limit
Investments Classified By Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Upper Limit
Investments Classified By Contractual Maturity Date [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years