Revenue from International Clients by Geographic Areas (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 179,815	$ 182,802	$ 601,042	$ 550,656
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		$ 95,087	$ 111,274	$ 351,330	$ 308,049
Geographic revenue percentage		53.00%	61.00%	59.00%	56.00%
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 8,722	$ 7,952	$ 30,243	$ 49,494
Geographic revenue percentage		5.00%	4.00%	5.00%	9.00%
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 18,485	$ 21,490	$ 68,003	$ 77,181
Geographic revenue percentage		10.00%	12.00%	11.00%	14.00%
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue	[1]	$ 28,100	$ 23,656	$ 76,958	$ 67,314
Geographic revenue percentage	[1]	16.00%	13.00%	13.00%	12.00%
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue		$ 29,421	$ 18,430	$ 74,508	$ 48,618
Geographic revenue percentage		16.00%	10.00%	12.00%	9.00%

[1] Includes Europe, the Middle East and Africa, but excludes the United Kingdom.