Reserves Textuals (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Reserves For Losses (Textuals) [Abstract]
Participation interests in Card Member receivables purchased from affiliates	$ 9,700.0	$ 6,000.0
Allowance for Card Member Receivables, Recoveries of Bad Debts	84.0	70.0
Allowance for Card Member Loans, Recoveries of Bad Debts	1.0	1.0
Participation Interest in Card Member Receivables Sold to Affiliates	$ 4,500.0	$ 2,600.0