UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Agency revenues	$ 2,332,371	$ 1,852,961	$ 4,117,684	$ 3,352,990
Merchant revenues	498,133	517,867	940,178	987,899
Advertising and other revenues	194,052	185,074	386,098	363,132
Total revenues	3,024,556	2,555,902	5,443,960	4,704,021
Cost of revenues	72,742	126,084	157,911	254,753
Gross profit	2,951,814	2,429,818	5,286,049	4,449,268
Operating expenses:
Performance advertising	1,147,589	920,763	2,128,362	1,700,672
Brand advertising	121,187	112,321	194,199	182,166
Sales and marketing	131,734	105,522	245,770	197,845
Personnel, including stock-based compensation of $66,879, $54,976, $125,827 and $120,976, respectively	385,708	332,654	736,738	641,005
General and administrative	141,634	112,642	277,181	225,687
Information technology	44,831	35,797	84,776	68,585
Depreciation and amortization	85,872	77,712	169,302	150,583
Total operating expenses	2,058,555	1,697,411	3,836,328	3,166,543
Operating income	893,259	732,407	1,449,721	1,282,725
Other income (expense):
Interest income	36,821	21,292	68,813	41,639
Interest expense	(60,942)	(50,290)	(116,659)	(97,184)
Foreign currency transactions and other	(6,021)	1,997	(11,148)	(10,931)
Impairment of cost-method investments	0	(12,858)	0	(63,208)
Total other expense	(30,142)	(39,859)	(58,994)	(129,684)
Earnings before income taxes	863,117	692,548	1,390,727	1,153,041
Income tax expense	142,908	111,910	214,895	197,979
Net income	$ 720,209	$ 580,638	$ 1,175,832	$ 955,062
Net income applicable to common stockholders per basic common share	$ 14.66	$ 11.71	$ 23.92	$ 19.25
Weighted-average number of basic common shares outstanding	49,131	49,604	49,161	49,617
Net income applicable to common stockholders per diluted common share	$ 14.39	$ 11.60	$ 23.49	$ 19.06
Weighted-average number of diluted common shares outstanding	50,056	50,059	50,049	50,105