Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Loan commitments available for funding	$ 36,808		$ 28,975
Commercial and standby letters of credit	3,451		3,007
Total unfunded credit commitments	40,259		31,982
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	398	$ 396	448	$ 513	$ 590	$ 305
Unfunded Credit Commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses: loans	$ 149	$ 120	$ 121	$ 101	$ 99	$ 67