Investments (Schedule of Investment Securities) (Details) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Other short-term investments	[1]	$ 8,048	$ 0
Investment securities	[1]	20,854	10,595
US Treasury Bill Securities [Member] | Asset Pledged as Collateral [Member]
Investment Holdings [Line Items]
Other short-term investments		5,909	0
US Treasury Bill Securities [Member] | Asset Not Pledged as Collateral [Member]
Investment Holdings [Line Items]
Other short-term investments	[2]	2,139	0
U.S. Treasury Securities [Member]
Investment Holdings [Line Items]
Investment securities		9,548	9,906
U.S. Treasury Securities [Member] | dfs_USTreasurySecuritiesMemberNonSept2020SLT [Member]
Investment Holdings [Line Items]
Derivative collateral		135	121
U.S. Treasury Securities [Member] | Asset Pledged as Collateral [Member]
Investment Holdings [Line Items]
Investment securities	[3]	5,618	121
U.S. Treasury Securities [Member] | Asset Not Pledged as Collateral [Member]
Investment Holdings [Line Items]
Investment securities		3,930	9,785
Residential Mortgage-Backed Securities - Agency [Member]
Investment Holdings [Line Items]
Investment securities	[4]	$ 11,306	$ 689

[1] Includes amounts related to a securities lending transaction in September 2020. See Note 2: Investments for additional information.

[2] (1)Includes U.S. Treasury bills with maturity dates greater than 90 days but less than one year at the time of acquisition.

[3] As of September 30, 2020 and December 31, 2019, includes $135 million and $121 million, respectively, of U.S. Treasury securities pledged as swap collateral other than amounts associated with the September 2020 securities lending transaction.

[4] Consists of residential mortgage-backed securities issued by Fannie Mae, Freddie Mac and Ginnie Mae.