Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Prepaid merchant bookings	$ 420	$ 226
Deferred merchant bookings	3,247	5,679
Deferred revenue	177	321
Deferred Merchant Bookings
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	2,476	4,898
Deferred merchant bookings recognized during period	3,429
Revenue recognized during period	569
Deferred Loyalty Rewards
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	771	781
Revenue recognized during period	333
Deferred Revenue
Disaggregation of Revenue [Line Items]
Revenue recognized during period	199
Deferred revenue	$ 177	$ 321