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, 2020 USD ($)
Investments Classified By Contractual Maturity Date [Line Items]
Total	$ 30,912,438
One Year or Less	34,584
After One Year to Five Years	3,532,784
After Five Years to Ten Years	2,666,731
After Ten Years	24,678,339
U.S. treasury securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	4,469,728
One Year or Less	10,092
After One Year to Five Years	3,532,784
After Five Years to Ten Years	926,852
After Ten Years	0
U.S. agency debentures
Investments Classified By Contractual Maturity Date [Line Items]
Total	237,307
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	237,307
After Ten Years	0
Foreign government debt securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	24,492
One Year or Less	24,492
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	13,503,681
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	13,503,681
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations - fixed rate
Investments Classified By Contractual Maturity Date [Line Items]
Total	8,106,564
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	0
After Ten Years	8,106,564
Agency-issued commercial mortgage-backed securities
Investments Classified By Contractual Maturity Date [Line Items]
Total	4,570,666
One Year or Less	0
After One Year to Five Years	0
After Five Years to Ten Years	1,502,572
After Ten Years	$ 3,068,094
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