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Retirement Plans - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Employee Benefits Disclosure [Line Items]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Employees contribution to 401(k) plan that is 100 % matched by employer 4.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent 4.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent 4.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Defined Contribution Plan, employer discretionary matching contribution percent 3.00%navg_DefinedContributionPlanEmployerDiscretionaryContributionPercentage 2.00%navg_DefinedContributionPlanEmployerDiscretionaryContributionPercentage 0.00%navg_DefinedContributionPlanEmployerDiscretionaryContributionPercentage
Defined Contribution Plan, employer discretionary matching contribution amount $ 1.6us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount $ 0.8us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount $ 0us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
401(k) plan expense 7.5us-gaap_DefinedContributionPlanCostRecognized 5.1us-gaap_DefinedContributionPlanCostRecognized 4.6us-gaap_DefinedContributionPlanCostRecognized
Retirement Savings Contributions      
Employee Benefits Disclosure [Line Items]      
401(k) plan expense 3.8us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_RetirementSavingsContributionsMember
2.7us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_RetirementSavingsContributionsMember
2.5us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_RetirementSavingsContributionsMember
Non Discretionary Matching Contributions      
Employee Benefits Disclosure [Line Items]      
401(k) plan expense 2.1us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_NonDiscretionaryMatchingContributionsMember
1.6us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_NonDiscretionaryMatchingContributionsMember
2.1us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= navg_NonDiscretionaryMatchingContributionsMember
Maximum      
Employee Benefits Disclosure [Line Items]      
Discretion of contribution to each eligible employee's 401(k) plan irrespective of the employees' contribution 4.00%navg_DefinedBenefitPlanEmployerDiscretionaryMatchPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
United States      
Employee Benefits Disclosure [Line Items]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 7.50%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_StatementGeographicalAxis
= country_US
   
Defined contribution plan employees vesting percentage beginning at the end of their second year 20.00%us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage
/ us-gaap_StatementGeographicalAxis
= country_US
   
Defined contribution plan additional employees vesting percentage being fully vested 6 years    
United Kingdom      
Employee Benefits Disclosure [Line Items]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 15.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_StatementGeographicalAxis
= country_GB
   
401(k) plan expense $ 2.6us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_StatementGeographicalAxis
= country_GB
$ 2.2us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_StatementGeographicalAxis
= country_GB
$ 1.9us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_StatementGeographicalAxis
= country_GB