Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Prepaid merchant bookings	$ 955	$ 389
Deferred merchant bookings	8,240	3,107
Deferred revenue	172	172
Deferred Merchant Bookings
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	7,500	2,300
Deferred merchant bookings recognized during period	1,400
Revenue recognized during period	216
Deferred Loyalty Rewards
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	757	769
Revenue recognized during period	213
Deferred Revenue
Disaggregation of Revenue [Line Items]
Revenue recognized during period	90
Deferred revenue	$ 172	$ 172