Legal Proceedings - 10-K (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Nov. 30, 2015	Jul. 31, 2015	Apr. 30, 2015	Mar. 31, 2015
Legal Reserve | Maximum
Loss Contingencies [Line Items]
Loss Contingency, Range of Possible Loss, Portion Not Accrued			$ 125
Interchange Litigation
Loss Contingencies [Line Items]
Litigation Settlement Amount	$ 900		6,600
Payments for Legal Settlements		$ 5,300
Conditional Return of Legal Settlements Paid Maximum Amount	$ 700
Litigation Settlement, Trigger Percent of Opt Out by Class Members for Return of Portion of Settlement Payments	15.00%
Litigation Settlement, Trigger Percent of Opt Out by Class Members to Allow Termination of Amended Settlement Agreement	25.00%
Interchange Litigation | Visa
Loss Contingencies [Line Items]
Litigation Settlement Amount	$ 600
Conditional Return of Legal Settlements Paid Maximum Amount	$ 467
Jo Ann Howard Pc Et Al V Cassity Et Al
Loss Contingencies [Line Items]
Alleged Present And Future Losses			$ 600
Compensatory damages							$ 356
Punitive Damages							$ 36
Prejudgement Interests						$ 179
Reduced Compensatory Damages				$ 289
DD Growth Premium Master Fund
Loss Contingencies [Line Items]
Alleged Claims For Loss In Net Asset Value Of Fund					$ 283
Alleged Claims For Certain Subscriptions Paid Into Fund					$ 134