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Commitments and Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Loss Contingencies [Line Items]      
Rent expense $ 13.2us-gaap_OperatingLeasesRentExpenseNet $ 11.5us-gaap_OperatingLeasesRentExpenseNet $ 10.5us-gaap_OperatingLeasesRentExpenseNet
Incentive awarded to the company by the State of Connecticut, total 11.5navg_StateGovernmentIncentiveAwardAmountAwarded    
Incentive awarded to the company by the State of Connecticut, non-interest bearing loans 8.0navg_StateGovernmentIncentiveAwardLoansAwarded    
Incentive awarded to the company by the State of Connecticut, grants 3.5navg_StateGovernmentIncentiveAwardGrantsAwarded    
Non-interest bearing incentive loan term 10 years    
Incentive awarded to the company by the State of Connecticut, revenue recognized 1.1us-gaap_DeferredRevenueRevenueRecognized 0.3us-gaap_DeferredRevenueRevenueRecognized  
Incentive awarded to the company by the State of Connecticut, deferred revenue $ 6.1us-gaap_DeferredRevenue $ 7.5us-gaap_DeferredRevenue  
Maximum      
Loss Contingencies [Line Items]      
Noncancellable operating leases expiration year 2023