UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Agency revenues	$ 3,523,706	$ 2,892,449	$ 7,641,390	$ 6,245,439
Merchant revenues	684,289	620,290	1,624,467	1,608,189
Advertising and other revenues	226,034	177,813	612,132	540,945
Total revenues	4,434,029	3,690,552	9,877,989	8,394,573
Cost of revenues	59,476	101,489	217,387	356,242
Gross profit	4,374,553	3,589,063	9,660,602	8,038,331
Operating expenses:
Performance advertising	1,224,345	1,040,149	3,352,707	2,740,821
Brand advertising	112,796	72,792	306,995	254,958
Sales and marketing	165,539	124,865	411,309	322,710
Personnel, including stock-based compensation of $66,421, $54,074, $192,248 and $175,050, respectively	483,438	347,610	1,220,176	988,615
General and administrative	142,823	114,586	420,004	340,273
Information technology	47,901	36,389	132,677	104,974
Depreciation and amortization	95,910	78,745	265,212	229,328
Impairment of goodwill	0	940,700	0	940,700
Total operating expenses	2,272,752	2,755,836	6,109,080	5,922,379
Operating income	2,101,801	833,227	3,551,522	2,115,952
Other income (expense):
Interest income	41,483	24,218	110,296	65,857
Interest expense	(66,338)	(55,480)	(182,997)	(152,664)
Foreign currency transactions and other	(10,101)	(4,431)	(21,249)	(15,362)
Impairment of cost-method investments	0	0	0	(63,208)
Total other expense	(34,956)	(35,693)	(93,950)	(165,377)
Earnings before income taxes	2,066,845	797,534	3,457,572	1,950,575
Income tax expense	346,454	291,517	561,349	489,496
Net income	$ 1,720,391	$ 506,017	$ 2,896,223	$ 1,461,079
Net income applicable to common stockholders per basic common share	$ 35.12	$ 10.24	$ 58.99	$ 29.49
Weighted-average number of basic common shares outstanding	48,981	49,420	49,100	49,548
Net income applicable to common stockholders per diluted common share	$ 34.43	$ 10.13	$ 57.85	$ 29.19
Weighted-average number of diluted common shares outstanding	49,972	49,975	50,064	50,048