(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
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 521,333	$ 424,148
Segment Reporting Information [Line Items]
Income from operations	64,658	48,697
Stock-based compensation expense	(21,856)	(16,596)
Depreciation and amortization expense	(10,200)	(8,176)
Interest and other income/(loss), net	3,076	(551)
Foreign exchange loss	(3,484)	(247)
Income before provision for income taxes	64,250	47,899
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	521,492	424,325
Segment Reporting Information [Line Items]
Income from operations	96,831	78,197
Unallocated amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	(159)	(177)
Segment Reporting Information [Line Items]
Other income included in segment revenues	(159)	(177)
Stock-based compensation expense	(21,856)	(16,596)
Non-corporate taxes	(1,728)	(2,560)
Professional fees	(1,769)	(1,916)
Depreciation and amortization expense	(2,140)	(1,769)
Bank charges	(606)	(589)
One-time charges and other acquisition-related expenses	(546)	(620)
Other operating expenses	$ (3,369)	$ (5,273)