Reserves (Details Textuals) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Reserves For Losses (Textuals) [Abstract]
Participation interests in Card Member receivables purchased from affiliates	$ 5,500.0	$ 6,000.0
Allowance for Card Member Receivables, Recoveries of Bad Debts	55.0	45.0
Allowance for Card Member Loans, Recoveries of Bad Debts	1.0	1.0
Participation Interest in Card Member Receivables Sold to Affiliates	$ 3,400.0	$ 2,600.0