UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
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
Total revenues	2,928,201	2,419,404
Cost of revenues		80,401
Gross profit		2,339,003
Operating expenses:
Performance marketing	1,106,207	982,172
Brand marketing	101,402	80,818
Sales and other expenses	165,800	109,599
Personnel, including stock-based compensation of $71,405 and $58,948, respectively	498,887	351,030
General and administrative	162,139	135,547
Information technology	60,441	39,945
Depreciation and amortization	103,090	83,430
Total operating expenses	2,197,966	1,782,541
Operating income	730,235	556,462
Other income (expense):
Interest income	46,879	31,992
Interest expense	(70,235)	(55,717)
Net unrealized gains on marketable equity securities	54,514	0
Foreign currency transactions and other	(8,056)	(5,127)
Total other income (expense)	23,102	(28,852)
Earnings before income taxes	753,337	527,610
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
Weighted-average number of basic common shares outstanding	48,349	49,192
Net income applicable to common stockholders per diluted common share	$ 12.34	$ 9.11
Weighted-average number of diluted common shares outstanding	49,205	50,025