Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies [Line Items]
Total rental expenses under operating lease agreements	$ 30.4	$ 28.3	$ 26.4
Northern Virginia Office Space
Commitments and Contingencies [Line Items]
Office space area leased under the agreement	233,000
Lease expiration date	2020-12
Deferred rent included in other long-term liabilities	18.9	20.1
Deferred rent included in current accrued expenses	$ 3.0	$ 2.8
Northern Virginia Office, Additional Office Space | 2014 Restructuring Plan
Commitments and Contingencies [Line Items]
Lease expiration date	2015-02
Office space ceased to be used	19,000