Revenue from International Clients by Geographic Areas (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 204,355	$ 162,403	$ 162,019	$ 153,918	$ 168,924	$ 137,631	$ 142,985	$ 140,464	$ 682,695	$ 590,004	$ 508,954
Geographic revenue percentage										100.00%	100.00%	100.00%
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										$ 379,936	$ 327,154	$ 278,945
Geographic revenue percentage										56.00%	55.00%	55.00%
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										$ 57,892	$ 32,642	$ 18,942
Geographic revenue percentage										8.00%	6.00%	4.00%
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										$ 96,314	$ 93,923	$ 83,134
Geographic revenue percentage										14.00%	16.00%	16.00%
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue	[1]									$ 87,240	$ 87,050	$ 82,743
Geographic revenue percentage	[1]									13.00%	15.00%	16.00%
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										$ 61,313	$ 49,235	$ 45,190
Geographic revenue percentage										9.00%	8.00%	9.00%

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