REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of adoption of accounting standard updates	$ 188,539
Retained earnings	14,975,706		$ 14,127,408	$ 13,938,869
Reduction in customer incentive liability	27,000
Customer incentive liability	78,800			104,700
Revenues	2,928,201	$ 2,419,404
Deferred revenue for merchant revenue transactions	219,900			$ 151,200
Revenue recognized related to the deferred revenue balance	85,600
Cash payments received from travelers reservations	192,200
Online Accommodation Reservation Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	2,500,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings	$ (133,224)		188,539
Deferred revenue for merchant revenue transactions			$ (32,400)
Revenues | Product Concentration Risk
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Concentration risk percentage	85.00%