Allowance for Credit Losses - Allowance for Credit Losses and Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions		Sep. 30, 2020		Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans		$ 1,084		$ 1,192	$ 467	$ 637	$ 676	$ 613	$ 541
Securities held-to-maturity	[2]	2	[1]		2
Other financial assets measured at amortized cost	[3]	3			3
Securities available-for-sale	[2]	2	[1]		3
Total allowance for credit losses		1,091			475
Liability for off-balance sheet credit exposures		$ 225		253	$ 158	158	$ 89	90	96
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				$ 253		$ 104		$ 90	$ 96

[1] As discussed in Note 21, "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 4, "Securities," for additional information regarding the allowance for credit losses associated with our security portfolios.

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