Summary of Significant Accounting Policies - Reserve for Merchant Credit Losses (Details) - Merchant credit losses - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Aggregate merchant credit losses incurred	$ 113	$ 40
Collateral held	1,200	510
Loss contingency accrual	$ 59	$ 34