COMMITMENTS AND CONTINGENCIES Operating Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating Lease, Types [Line Items]
2018	$ 149,699
2019	141,305
2020	117,258
2021	86,902
2022	54,704
After 2022	207,603
Total	757,471
Office Leases [Member]
Operating Lease, Types [Line Items]
Operating Leases, Rent Expense, Net	96,100	$ 77,300	$ 64,800
Data Center Space, Leases [Member]
Operating Lease, Types [Line Items]
Operating Leases, Rent Expense, Net	$ 23,700	$ 22,200	$ 21,600
Norwalk, Connecticut [Member] | Parent Company [Member]
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	90,000
Amsterdam, Netherlands [Member] | Booking.com
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	258,000
Stamford, Connecticut [Member] | KAYAK
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	18,000
Bangkok, Thailand [Member] | agoda
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	144,000
San Francisco, California [Member] | OpenTable
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	60,000
Manchester, England [Member] | Rentalcars.com
Operating Lease, Types [Line Items]
Area of Real Estate Property | ft²	45,000