Allowance for Credit Losses and Reserve for Unfunded Lending Commitments - Summary of Loss Sharing Arrangements (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision (benefit) for credit losses	$ (342)	$ 331	$ (2,325)	$ 10,000
Loss sharing agreements
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Expected reimbursement from loss sharing partners, beginning balance	1,711	2,433	2,159	2,166
Amounts due from partners which reduced net charge-offs	(80)	(212)	(349)	(807)
Provision (benefit) for credit losses	(181)	121	(360)	983
Expected reimbursement from loss sharing partners, ending balance	$ 1,450	$ 2,342	$ 1,450	$ 2,342