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COMMITMENTS AND CONTINGENCIES (Details) ₺ in Millions, € in Millions, £ in Millions, $ in Millions	1 Months Ended												3 Months Ended
	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 ($)	Mar. 31, 2021 EUR (€)	Mar. 31, 2021 TRY (₺)	Mar. 31, 2021 EUR (€)	Mar. 31, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)	Sep. 30, 2016 EUR (€)
Commitments and Contingencies
Unrecognized tax benefits	$ 100												$ 100					$ 84
Booking.com \| Headquarters
Commitments and Contingencies
Contractual obligation	52								$ 318				52			€ 44				€ 270
Acquisition of land use rights									$ 48	€ 43
Booking.com \| Headquarters \| Headquarters Vendors
Commitments and Contingencies
Contractual obligation	37												37			32
Booking.com \| Headquarters \| Ground Lease
Commitments and Contingencies
Remaining lease payments	$ 82												$ 82			€ 70
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	$ 89												$ 89				£ 65
Potential Fine under Contractual Parity Agreement
Commitments and Contingencies
Accruals for loss contingencies	22												22					23
French Tax Audit
Commitments and Contingencies
Assessed taxes including interest and penalties			$ 83	€ 70							$ 403	€ 356
Payment required to appeal a litigation matter or avoid collection enforcement					$ 403	€ 356
Unrecognized tax benefits																		58	€ 50
Estimated reasonably possible loss in excess of amount accrued	$ 24												24			€ 20
French Tax Assessment, transfer taxes
Commitments and Contingencies
Assessed taxes including interest and penalties			46	39
Italian Tax Audit
Commitments and Contingencies
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							$ 56	€ 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			$ 68	€ 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
Turkish Tax Audit
Commitments and Contingencies
Assessed taxes including interest and penalties													94		₺ 778
Travel Transaction Related Taxes
Commitments and Contingencies
Accruals for loss contingencies	57												57					$ 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	300												300
Unfavorable Regulatory Action \| Maximum
Commitments and Contingencies
Estimated possible fine per incident	$ 24												$ 24			€ 20