REVENUE RECOGNITION - Cumulative Effect of Revenue Accounting Changes on the Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Current assets:
Accounts receivable, net	$ 1,715,055	$ 1,423,125	$ 1,217,801
Current liabilities:
Accounts payable	1,053,509	839,167	667,523
Accrued expenses and other current liabilities	1,418,603	1,183,354	1,138,980
Deferred merchant bookings	1,802,182	778,808	980,455
Deferred income taxes	502,581	483,553	481,139
Stockholders' equity:
Retained earnings	15,953,078	14,127,408	13,938,869
Adjustments | Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	(148,982)	205,324
Current liabilities:
Accounts payable	(110,170)	171,644
Accrued expenses and other current liabilities	(8,883)	44,374
Deferred merchant bookings	145,297	(201,647)
Deferred income taxes	(285)	2,414
Stockholders' equity:
Retained earnings	(139,366)	$ 188,539
As Adjusted (Previous Revenue Standard)
Current assets:
Accounts receivable, net	1,566,073		1,217,801
Current liabilities:
Accounts payable	943,339		667,523
Accrued expenses and other current liabilities	1,409,720		1,138,980
Deferred merchant bookings	1,947,479		980,455
Deferred income taxes	502,296		481,139
Stockholders' equity:
Retained earnings	$ 15,813,712		$ 13,938,869