Nature of Expenses Allocated by Function - Sales and Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Income Statements, Captions [Line Items]
Personnel expenses	$ (332,353)	$ (246,171)	$ (189,418)
Personnel expense excluding equity awards compensation expense and research tax credit	(220,317)	(170,079)	(128,737)
Equity awards compensation expense	(43,259)	(23,989)	(19,600)
Provision for doubtful accounts	(9,898)	(2,660)	(2,248)
Total sales and operations expenses	(282,853)	(229,530)	(176,927)
Sales And Operations Expense
Condensed Income Statements, Captions [Line Items]
Personnel expenses	(185,065)	(150,426)	(119,609)
Personnel expense excluding equity awards compensation expense and research tax credit	(168,227)	(138,748)	(107,319)
Equity awards compensation expense	(16,838)	(11,678)	(12,290)
Other cash operating expenses	(84,127)	(71,034)	(52,077)
Subcontracting and other headcount related costs	(22,460)	(20,856)	(17,363)
Rent and facilities costs	(29,968)	(25,542)	(15,684)
Marketing costs	(15,225)	(12,478)	(9,443)
Other	(16,474)	(12,158)	(9,587)
Other non-cash operating expenses	(13,661)	(8,070)	(5,241)
Depreciation and amortization	(7,757)	(5,178)	(3,664)
Provision for doubtful accounts	(5,433)	(2,660)	(1,342)
Net change in other provisions	$ (471)	$ (232)	$ (235)