(Reconciliation of Segment Revenues to Consolidated Revenues and Segment Operating Profit to Consolidated Income Before Provision for/(Benefit from) Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 551,587	$ 445,647	$ 1,072,920	$ 869,795
Segment Reporting Information [Line Items]
Income from operations	72,882	54,237	137,540	102,934
Stock-based compensation expense			(37,553)	(31,406)
Depreciation and amortization expense	(11,028)	(8,962)	(21,228)	(17,138)
Interest and other income, net	1,190	1,052	4,266	501
Foreign exchange (loss)/gain	(3,562)	1,830	(7,046)	1,583
Income before provision for income taxes	70,510	57,119	134,760	105,018
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	551,692	445,978	1,073,184	870,303
Segment Reporting Information [Line Items]
Income from operations	102,678	80,110	199,509	158,307
Unallocated amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	(105)	(331)	(264)	(508)
Segment Reporting Information [Line Items]
Other income included in segment revenues	(105)	(331)	(264)	(508)
Stock-based compensation expense	(15,697)	(14,810)	(37,553)	(31,406)
Non-corporate taxes	(2,438)	(2,418)	(4,166)	(4,978)
Professional fees	(555)	(1,400)	(2,324)	(3,316)
Depreciation and amortization expense	(2,603)	(1,974)	(4,743)	(3,743)
Bank charges	(686)	(579)	(1,292)	(1,168)
One-time charges and other acquisition-related expenses	(815)	(1,241)	(1,361)	(1,861)
Other operating expenses	$ (6,897)	$ (3,120)	$ (10,266)	$ (8,393)