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, 2019 USD ($)
Investments Classified By Contractual Maturity Date [Line Items]
Total	$ 14,014,919
One Year or Less	1,845,009
After One Year to Five Years	1,890,778
After Five Years to Ten Years	4,012,540
After Ten Years	6,266,592
U.S. treasury securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	6,894,010
One Year or Less	1,835,971
After One Year to Five Years	1,890,778
After Five Years to Ten Years	3,167,261
After Ten Years	0
U.S. agency debentures
Investments Classified By Contractual Maturity Date [Line Items]
Total	99,547
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	99,547
After Ten Years	0
Foreign government debt securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	9,038
One Year or Less	9,038
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	0
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	4,148,791
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	4,148,791
Residential mortgage-backed securities | Fixed rate
Investments Classified By Contractual Maturity Date [Line Items]
Total	1,538,343
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	1,858
After Ten Years	1,536,485
Agency-issued commercial mortgage-backed securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	1,325,190
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	743,874
After Ten Years	$ 581,316
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