Pledged and Restricted Assets - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 47,500,000,000	$ 42,100,000,000
Financial instruments owned and pledged as collateral, amount eligible to be repledged by counterparty	33,500,000	487,200,000
Amount of securities repledged or sold collateral	0	0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	400,000,000	1,500,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 5,700,000,000	$ 8,500,000,000