Revenue from International Clients by Geographic Areas (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 182,802	$ 162,403	$ 550,656	$ 478,340
Geographic revenue percentage		100.00%	100.00%	100.00%	100.00%
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 111,274	$ 89,640	$ 308,049	$ 267,671
Geographic revenue percentage		61.00%	55.00%	56.00%	56.00%
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 7,952	$ 6,602	$ 49,494	$ 46,278
Geographic revenue percentage		4.00%	4.00%	9.00%	10.00%
U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 21,490	$ 19,491	$ 77,181	$ 60,867
Geographic revenue percentage		12.00%	12.00%	14.00%	13.00%
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue	[1]	$ 23,656	$ 20,672	$ 67,314	$ 55,376
Geographic revenue percentage	[1]	13.00%	13.00%	12.00%	11.00%
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 18,430	$ 25,998	$ 48,618	$ 48,148
Geographic revenue percentage		10.00%	16.00%	9.00%	10.00%

[1] Includes Europe, the Middle East and Africa ("Other EMEA").