DIscontinued Operations (Narrative) (Details) $ in Millions		1 Months Ended	3 Months Ended				5 Months Ended	7 Months Ended	12 Months Ended
	May 09, 2018 USD ($)	Jan. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	May 31, 2018 segment	Dec. 31, 2018 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal			$ 4	$ 20	$ 432	$ 0			$ 456	$ 0	$ 0
Gain on disposal, tax									139
Separation and related costs									223	26	$ 0
Number of reportable segments | segment							3	2
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Separation and related costs									111	$ 40
European vacation rentals business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of business	$ 1,060
Guarantees, post-closing credit support, escrow deposit	$ 46
Secured bonding facility and perpetual guarantee			$ 46					$ 46	$ 46
Knights Inn Hotels
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of business					27
Gain on disposal					$ 23
Subsequent Event | European vacation rentals business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Post-closing adjustments		$ 27