UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Agency revenues	$ 2,566,293		$ 4,679,632
Agency revenues		$ 2,332,371		$ 4,117,684
Merchant revenues	709,864		1,236,331
Merchant revenues		498,133		940,178
Advertising and other revenues	260,937	194,052	549,332	386,098
Total revenues	3,537,094	3,024,556	6,465,295	5,443,960
Cost of revenues		67,425		147,826
Gross profit		2,957,131		5,296,134
Operating expenses:
Performance marketing	1,141,893	1,151,343	2,248,100	2,133,515
Brand marketing	123,734	130,321	225,136	211,139
Sales and other expenses	203,593	121,915	369,393	231,514
Personnel, including stock-based compensation of $74,540, $66,879, $145,945 and $125,827, respectively	522,250	385,708	1,021,137	736,738
General and administrative	158,753	143,882	320,892	279,429
Information technology	58,950	44,831	119,391	84,776
Depreciation and amortization	106,666	85,872	209,756	169,302
Total operating expenses	2,315,839	2,063,872	4,513,805	3,846,413
Operating income	1,221,255	893,259	1,951,490	1,449,721
Other (expense) income:
Interest income	45,690	36,821	92,569	68,813
Interest expense	(64,837)	(60,942)	(135,072)	(116,659)
Net unrealized gains on marketable equity securities	21,849	0	76,363	0
Foreign currency transactions and other	(15,046)	(6,021)	(23,102)	(11,148)
Total other (expense) income	(12,344)	(30,142)	10,758	(58,994)
Earnings before income taxes	1,208,911	863,117	1,962,248	1,390,727
Income tax expense	231,539	142,908	377,666	214,895
Net income	$ 977,372	$ 720,209	$ 1,584,582	$ 1,175,832
Net income applicable to common stockholders per basic common share	$ 20.34	$ 14.66	$ 32.87	$ 23.92
Weighted-average number of basic common shares outstanding	48,055	49,131	48,202	49,161
Net income applicable to common stockholders per diluted common share	$ 20.13	$ 14.39	$ 32.42	$ 23.49
Weighted-average number of diluted common shares outstanding	48,550	50,056	48,877	50,049