UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Agency revenues	$ 1,852,961	$ 1,582,153	$ 3,352,990	$ 2,781,501
Merchant revenues	517,867	546,013	987,899	1,040,688
Advertising and other revenues	185,074	152,231	363,132	298,902
Total revenues	2,555,902	2,280,397	4,704,021	4,121,091
Cost of revenues	126,084	187,491	254,753	355,949
Gross profit	2,429,818	2,092,906	4,449,268	3,765,142
Operating expenses:
Performance advertising	920,763	758,690	1,700,672	1,392,234
Brand advertising	112,321	78,431	182,166	151,685
Sales and marketing	105,522	94,523	197,845	176,467
Personnel, including stock-based compensation of $54,976, $60,164, $120,976 and $114,172, respectively	332,654	289,156	641,005	548,140
General and administrative	112,642	98,945	225,687	199,123
Information technology	35,797	27,156	68,585	52,517
Depreciation and amortization	77,712	67,674	150,583	132,676
Total operating expenses	1,697,411	1,414,575	3,166,543	2,652,842
Operating income	732,407	678,331	1,282,725	1,112,300
Other income (expense):
Interest income	21,292	13,037	41,639	24,633
Interest expense	(50,290)	(41,547)	(97,184)	(75,026)
Foreign currency transactions and other	1,997	(1,444)	(10,931)	(6,287)
Impairment of cost-method investments	(12,858)	0	(63,208)	0
Total other expense	(39,859)	(29,954)	(129,684)	(56,680)
Earnings before income taxes	692,548	648,377	1,153,041	1,055,620
Income tax expense	111,910	131,345	197,979	205,261
Net income	$ 580,638	$ 517,032	$ 955,062	$ 850,359
Net income applicable to common stockholders per basic common share	$ 11.71	$ 10.02	$ 19.25	$ 16.43
Weighted-average number of basic common shares outstanding	49,604	51,589	49,617	51,748
Net income applicable to common stockholders per diluted common share	$ 11.60	$ 9.94	$ 19.06	$ 16.27
Weighted-average number of diluted common shares outstanding	50,059	52,038	50,105	52,253