Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Prepaid merchant bookings	$ 579	$ 226
Prepaid merchant bookings, reserve	11
Deferred merchant bookings	4,632	5,679
Deferred revenue	197	321
Deferred Merchant Bookings
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	3,842	4,898
Deferred merchant bookings recognized during period	3,173
Revenue recognized during period	506
Deferred Loyalty Rewards
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	790	781
Revenue recognized during period	209
Deferred Revenue
Disaggregation of Revenue [Line Items]
Revenue recognized during period	159
Deferred revenue	$ 197	$ 321