Allowance for Credit Losses - Allowance for Credit Losses and Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Credit Loss [Abstract]
Loans		$ 853	$ 1,015	$ 931
Securities held-to-maturity	[1]	2	2
Other financial assets measured at amortized cost	[2]	2	2
Securities available-for-sale	[1]	1	1
Total allowance for credit losses		858	1,020
Liability for off-balance sheet credit exposures		$ 155	$ 237		$ 158

[1] See Note 4, "Securities," for additional information regarding the allowance for credit losses associated with our security portfolios.

[2] Primarily includes accrued interest receivables and customer acceptances.