CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Agency revenues	$ 9,714,126	$ 7,982,116	$ 6,527,898
Merchant revenues	2,133,017	2,048,005	2,082,973
Advertising and other revenues	833,939	712,885	613,116
Total revenues	12,681,082	10,743,006	9,223,987
Cost of revenues	250,537	428,314	632,180
Gross profit	12,430,545	10,314,692	8,591,807
Operating expenses:
Performance advertising	4,141,771	3,479,287	2,738,218
Brand advertising	391,584	295,698	273,704
Sales and marketing	561,958	435,225	353,221
Personnel, including stock-based compensation of $260,910, $249,574 and $247,395, respectively	1,659,581	1,350,032	1,166,226
General and administrative	585,541	455,909	415,420
Information technology	189,344	142,393	113,617
Depreciation and amortization	362,774	309,135	272,494
Impairment of goodwill	0	940,700	0
Total operating expenses	7,892,553	7,408,379	5,332,900
Operating income	4,537,992	2,906,313	3,258,907
Other income (expense):
Interest income	157,194	94,946	55,729
Interest expense	(253,976)	(207,900)	(160,229)
Foreign currency transactions and other	(35,291)	(16,913)	(26,087)
Impairment of cost-method investments	(7,597)	(63,208)	0
Total other expense	(139,670)	(193,075)	(130,587)
Earnings before income taxes	4,398,322	2,713,238	3,128,320
Income tax expense	2,057,557	578,251	576,960
Net income	$ 2,340,765	$ 2,134,987	$ 2,551,360
Net income applicable to common stockholders per basic common share	$ 47.78	$ 43.14	$ 50.09
Weighted-average number of basic common shares outstanding	48,994	49,491	50,940
Net income applicable to common stockholders per diluted common share	$ 46.86	$ 42.65	$ 49.45
Weighted-average number of diluted common shares outstanding	49,954	50,063	51,593