Divestitures (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred merchant bookings		$ 3,247	$ 5,679
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Held-for-sale assets		60
Held-for-sale assets, cash		17
Held-for-sale assets, accounts receivable		18
Held-for-sale assets, prepaid expenses and other current assets		15
Held-for-sale liabilities		68
Held-for-sale liabilities, accrued expenses and other current liabilities		17
Deferred merchant bookings		$ 36
Disposal Group, Held-for-sale, Not Discontinued Operations | Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposition of business	$ (20)