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Tax Effects of Temporary Differences That Give Rise to Federal, Foreign, State and Local Deferred Tax Assets and Deferred Tax Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Deferred tax assets:    
Loss reserve discount $ 24,820us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves $ 27,822us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves
Unearned premiums 29,080us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve 25,706us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve
Compensation related 10,586us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits 6,782us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits
State and local net deferred tax assets 777us-gaap_DeferredTaxAssetsStateTaxes 555us-gaap_DeferredTaxAssetsStateTaxes
Other 1,834us-gaap_DeferredTaxAssetsOther 3,889us-gaap_DeferredTaxAssetsOther
Total gross deferred tax assets 67,097us-gaap_DeferredTaxAssetsGross 64,754us-gaap_DeferredTaxAssetsGross
Less: Valuation allowance (777)us-gaap_DeferredTaxAssetsValuationAllowance (555)us-gaap_DeferredTaxAssetsValuationAllowance
Total deferred tax assets 66,320us-gaap_DeferredTaxAssetsNet 64,199us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:    
Net unrealized gains/losses on securities (24,832)us-gaap_DeferredTaxLiabilitiesInvestments (9,119)us-gaap_DeferredTaxLiabilitiesInvestments
Deferred acquisition costs (22,120)us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost (19,258)us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost
Net unrealized foreign exchange (4,470)us-gaap_DeferredTaxLiabilitiesUnrealizedCurrencyTransactionGains (3,516)us-gaap_DeferredTaxLiabilitiesUnrealizedCurrencyTransactionGains
Other (2,787)us-gaap_DeferredTaxLiabilitiesOther (4,119)us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities (67,787)us-gaap_DeferredIncomeTaxLiabilities (40,393)us-gaap_DeferredIncomeTaxLiabilities
Net deferred income tax (liability) (1,467)us-gaap_DeferredTaxLiabilities  
Net deferred income tax asset   23,806us-gaap_DeferredTaxAssetsLiabilitiesNet
Lloyd's    
Deferred tax liabilities:    
Lloyd's year of account deferral $ (13,578)us-gaap_DeferredTaxLiabilitiesDeferredExpense
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember		 $ (4,381)us-gaap_DeferredTaxLiabilitiesDeferredExpense
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember