Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class, Including Delinquency Status of those that Continue to Accrue Interest and are Nonperforming (Parenthetical) (Detail) - Government National Mortgage Association [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 days past due purchased from Government National Mortgage Association mortgage pools, classified as current	$ 1,500		$ 1,400
Loans 90 days or more past due purchased from Government National Mortgage Association mortgage pools, classified as current	1,700		$ 1,800
Interest income on nonperforming loans	$ 3	$ 5