UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Agency revenues	$ 1,785,313	$ 1,500,029
Merchant revenues	442,045	470,032
Advertising and other revenues	192,046	178,058
Total revenues	2,419,404	2,148,119
Cost of revenues	85,169	128,669
Gross profit	2,334,235	2,019,450
Operating expenses:
Performance advertising	980,773	779,909
Brand advertising	73,012	69,845
Sales and marketing	114,036	92,323
Personnel, including stock-based compensation of $58,948 and $66,000, respectively	351,030	308,351
General and administrative	135,547	113,045
Information technology	39,945	32,788
Depreciation and amortization	83,430	72,871
Total operating expenses	1,777,773	1,469,132
Operating income	556,462	550,318
Other income (expense):
Interest income	31,992	20,347
Interest expense	(55,717)	(46,894)
Foreign currency transactions and other	(5,127)	(12,928)
Impairment of cost-method investment	0	(50,350)
Total other expense	(28,852)	(89,825)
Earnings before income taxes	527,610	460,493
Income tax expense	71,987	86,069
Net income	$ 455,623	$ 374,424
Net income applicable to common stockholders per basic common share	$ 9.26	$ 7.54
Weighted-average number of basic common shares outstanding	49,192	49,630
Net income applicable to common stockholders per diluted common share	$ 9.11	$ 7.47
Weighted-average number of diluted common shares outstanding	50,025	50,129