Revenue from International Clients by Geographic Areas (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue	$ 168,924	$ 137,631	$ 142,985	$ 140,464	$ 153,382	[1]	$ 122,011	$ 117,315	$ 116,246	$ 590,004		$ 508,954		$ 461,710
Geographic revenue percentage										100.00%		100.00%		100.00%
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										327,154		278,945		250,022
Geographic revenue percentage										55.00%		55.00%		54.00%
Other Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										32,642		18,942		27,741
Geographic revenue percentage										6.00%		4.00%		6.00%
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										93,923		83,134		87,135
Geographic revenue percentage										16.00%		16.00%		19.00%
Other EMEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										87,050	[2]	82,743	[2]	70,583	[2]
Geographic revenue percentage										15.00%	[2]	16.00%	[2]	15.00%	[2]
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Geographic revenue										$ 49,235		$ 45,190		$ 26,229
Geographic revenue percentage										8.00%		9.00%		6.00%

[1] Reflects the Company's acquisition of Antenna on October 9, 2013. See Note 8 "Acquisitions" for pro forma information for the year ended December 31, 2013.

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