REVENUE RECOGNITION - Impacts of Adopting Current Revenue Standard (Details) - USD ($) $ / shares in Units, $ 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
Current assets:
Accounts receivable, net	1,530,321		$ 1,423,125	$ 1,217,801
Prepaid expenses and other current assets	1,111,753			415,527
Current liabilities:
Accounts payable	841,274		839,167	667,523
Accrued expenses and other current liabilities	1,238,815		1,183,354	1,138,980
Deferred merchant bookings	1,425,649		778,808	980,455
Deferred income taxes	481,220		483,553	481,139
Stockholders' equity:
Retained earnings	14,975,706		14,127,408	13,938,869
Accumulated other comprehensive income	56,406			236,982
Consolidated Statements of Operations
Agency revenues	2,113,339
Agency revenues		$ 1,785,313
Merchant revenues	526,467
Merchant revenues		442,045
Advertising and other revenues	288,395	192,046
Cost of revenues		80,401
Operating expenses:
Performance marketing	1,106,207	982,172
Sales and other expenses	165,800	109,599
General and administrative	162,139	135,547
Foreign currency transactions and other	(8,056)	(5,127)
Income tax expense	146,127	71,987
Net income	$ 607,210	$ 455,623
Net income applicable to common stockholders per basic common share	$ 12.56	$ 9.26
Net income applicable to common stockholders per diluted common share	$ 12.34	$ 9.11
Adjustments | Accounting Standards Update 2014-09
Current assets:
Accounts receivable, net	$ (152,477)		205,324
Prepaid expenses and other current assets	19,531
Current liabilities:
Accounts payable	(104,172)		171,644
Accrued expenses and other current liabilities	(8,897)		44,374
Deferred merchant bookings	117,367		(201,647)
Deferred income taxes	(984)		2,414
Stockholders' equity:
Retained earnings	(133,224)		$ 188,539
Accumulated other comprehensive income	(3,310)
Consolidated Statements of Operations
Agency revenues	55,987
Merchant revenues	61,540
Advertising and other revenues	91
Cost of revenues	45,660
Operating expenses:
Performance marketing	2,394
Sales and other expenses	205
General and administrative	801
Foreign currency transactions and other	1,855
Income tax expense	15,098
Net income	$ 55,315
Net income applicable to common stockholders per basic common share	$ 1.14
Net income applicable to common stockholders per diluted common share	$ 1.12
As Adjusted (Previous Revenue Standard) [Member]
Current assets:
Accounts receivable, net	$ 1,377,844			1,217,801
Prepaid expenses and other current assets	1,131,284
Current liabilities:
Accounts payable	737,102			667,523
Accrued expenses and other current liabilities	1,229,918			1,138,980
Deferred merchant bookings	1,543,016			980,455
Deferred income taxes	480,236			481,139
Stockholders' equity:
Retained earnings	14,842,482			$ 13,938,869
Accumulated other comprehensive income	53,096
Consolidated Statements of Operations
Agency revenues	2,169,326
Merchant revenues	588,007
Advertising and other revenues	288,486
Cost of revenues	45,660
Operating expenses:
Performance marketing	1,108,601
Sales and other expenses	166,005
General and administrative	162,940
Foreign currency transactions and other	(6,201)
Income tax expense	161,225
Net income	$ 662,525
Net income applicable to common stockholders per basic common share	$ 13.70
Net income applicable to common stockholders per diluted common share	$ 13.46