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COMMITMENTS AND CONTINGENCIES (Details) ₺ in Millions, € in Millions, £ in Millions, $ in Millions	1 Months Ended																			3 Months Ended		9 Months Ended
	Oct. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 EUR (€)	Aug. 31, 2021 USD ($)	Aug. 31, 2021 EUR (€)	Jun. 30, 2021 USD ($)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 USD ($)	Mar. 31, 2021 EUR (€)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Jan. 31, 2019 USD ($)	Jan. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 EUR (€)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 EUR (€)	Mar. 31, 2021 USD ($)	Mar. 31, 2021 EUR (€)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 EUR (€)	Sep. 30, 2021 TRY (₺)	Sep. 30, 2021 EUR (€)	Sep. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 EUR (€)	Sep. 30, 2016 EUR (€)
Commitments and Contingencies
Unrecognized tax benefits		$ 97																				$ 97					$ 84
Booking.com \| Headquarters
Commitments and Contingencies
Contractual obligation		25														$ 313						25			€ 22						€ 270
Acquisition of land use rights																$ 48	€ 43
Booking.com \| Headquarters \| Headquarters Vendors
Commitments and Contingencies
Contractual obligation		31																				31			27
Booking.com \| Headquarters \| Ground Lease
Commitments and Contingencies
Remaining lease payments		$ 79																				$ 79			€ 69
Rentalcars.com \| Headquarters \| Manchester, England
Commitments and Contingencies
Lease term of operating leases which have not commenced at period end		13 years																				13 years			13 years	13 years
Rentalcars.com \| Headquarters \| Operating Lease Obligations \| Manchester, England
Commitments and Contingencies
Contractual obligation		$ 71																				$ 71				£ 52
Potential Fine under Contractual Parity Agreement
Commitments and Contingencies
Accruals for loss contingencies		17																				17			€ 14		23	€ 18
French Tax Audit
Commitments and Contingencies
Unrecognized tax benefits																											59	50
Estimated reasonably possible loss in excess of amount accrued		$ 23																				23			€ 20
French Tax Audit \| Tax Year 2013
Commitments and Contingencies
Assessed taxes including interest and penalties										$ 81	€ 70
French Tax Audit \| Tax Years 2006 Through 2012
Commitments and Contingencies
Assessed taxes including interest and penalties																		$ 403	€ 356
Payment required to appeal a litigation matter or avoid collection enforcement												$ 403	€ 356
French Tax Assessment, transfer taxes \| Tax Years 2011 Through 2015
Commitments and Contingencies
Assessed taxes including interest and penalties										45	39
Italian Tax Audit
Commitments and Contingencies
Expected required prepayment deposit or bank guarantees																						$ 70	€ 60
Net unrecognized tax benefit recognized																				$ 16	€ 13
Income taxes, percent of tax and interest expected to be paid related to pending proceedings		33.00%																				33.00%			33.00%	33.00%
Italian Tax Audit \| Other assets, net
Commitments and Contingencies
Unrecognized tax benefits		$ 6																				$ 6			€ 5		6	€ 5
Italian Tax Audit \| Tax Year 2013
Commitments and Contingencies
Assessed taxes including interest and penalties														$ 55	€ 48
Payment required to appeal a litigation matter or avoid collection enforcement										11	10
Italian Tax Audit \| Tax Year 2014
Commitments and Contingencies
Assessed taxes including interest and penalties										$ 67	€ 58
Italian Tax Audit \| Tax Year 2015
Commitments and Contingencies
Assessed taxes including interest and penalties								$ 37	€ 31
Italian Tax Audit \| Tax Years 2013 And 2014
Commitments and Contingencies
Unrecognized tax benefits																													$ 5	€ 4
Italian Tax Audit \| Tax Years 2013 through 2019
Commitments and Contingencies
Loss Contingency, Tax, Recommended Assessment						$ 178	€ 154
Italian Tax Audit \| Tax Years 2016 Through 2018
Commitments and Contingencies
Loss Contingency, Tax, Assessment				$ 132	€ 114
Turkish Tax Audit
Commitments and Contingencies
Assessed taxes including interest and penalties																						90		₺ 801
Swiss Tax Assessment \| Tax Years 2015 Through 2020
Commitments and Contingencies
Loss Contingency, Tax, Assessment		54	€ 50
Hawaii Tax Assessment \| Tax Years 2010 Through 2020 \| Subsequent Event
Commitments and Contingencies
Loss Contingency, Tax, Recommended Assessment	$ 20
Travel Transaction Related Taxes
Commitments and Contingencies
Accruals for loss contingencies		54																				54					$ 59
Estimated reasonably possible loss in excess of amount accrued		20																				20
Pension-related litigation
Commitments and Contingencies
Estimated reasonably possible loss in excess of amount accrued		322																				322			278
Unfavorable Regulatory Action \| Maximum
Commitments and Contingencies
Estimated possible fine per incident		$ 23																				$ 23			€ 20