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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2014
USD ($)
ft²
Sep. 30, 2015
USD ($)
Northern Virginia Office Space    
Commitments and Contingencies [Line Items]    
Office space area leased under the agreement | ft² 233,000  
Lease expiration date 2020-12  
Deferred rent included in other long-term liabilities $ 18.9 $ 16.6
Deferred rent included in current accrued expenses $ 3.0 $ 3.3
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 | ft² 19,000