Allowance for Credit Losses - Allowance for Credit Losses and Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions		Dec. 31, 2020		Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans		$ 1,015		$ 467	$ 637	$ 541	$ 681
Securities held-to-maturity	[2]	2	[1]	2
Other financial assets measured at amortized cost	[3]	2		3
Securities available-for-sale	[2]	1	[1]	3
Total allowance for credit losses		1,020		475
Liability for off-balance sheet credit exposures		$ 237		$ 158	158	96	106
As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans					637
Securities held-to-maturity	[2]				0
Other financial assets measured at amortized cost	[3]				0
Securities available-for-sale	[2]				0
Total allowance for credit losses					637
Liability for off-balance sheet credit exposures					$ 104	$ 96	$ 106

[1] As discussed in Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," beginning January 1, 2020, an allowance for credit losses is recognized for debt securities while, prior to January 1, 2020, debt securities were assessed for other-than-temporary impairment. At December 31, 2019, we did not consider any of our debt securities to be other-than-temporarily impaired.

[2] See Note 5, "Securities," for additional information regarding the allowance for credit losses associated with our security portfolios.

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