Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Allowance for Credit Losses: Unfunded Credit Commitments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	$ 392	$ 549	$ 448	$ 305
Provision of credit losses	16	52	50	301
Allowance for credit loss, ending balance	396	590	396	590
Cumulative adjustment for the adoption of amendment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				25
Unfunded Credit Commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	105	85	121	67
Provision of credit losses	15	14	(1)	9
Allowance for credit loss, ending balance	120	99	120	99
Unfunded Credit Commitments | Cumulative adjustment for the adoption of amendment
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	$ 0	$ 0	$ 0	$ 23