Reserves for Credit Losses - Changes in Card Member Loans Reserve for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provisions	$ 242.0	$ 230.0	$ 249.0
Cumulative Effect, Period of Adoption, Adjustment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Increase related to adoption of CECL methodology				$ 7.0
Card Member Loans [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1	10.0	5.0	5.0
Provisions	18.0	14.0	6.0
Net write-offs	(15.0)	(9.0)	(6.0)
Balance, December 31	20.0	10.0	5.0
Net of recoveries	2.0	1.6	$ 1.8
Card Member Loans [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, January 1	$ 17.0
Balance, December 31		$ 17.0