Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Prepaid merchant bookings	$ 533	$ 226
Deferred merchant bookings	5,905	5,679
Deferred revenue	221	321
Deferred Merchant Bookings
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	5,116	4,898
Deferred merchant bookings recognized during period	2,880
Revenue recognized during period	442
Deferred Loyalty Rewards
Disaggregation of Revenue [Line Items]
Deferred merchant bookings	789	781
Revenue recognized during period	158
Deferred Revenue
Disaggregation of Revenue [Line Items]
Revenue recognized during period	109
Deferred revenue	$ 221	$ 321