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Ceded Reinsurance - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Entity
Dec. 31, 2013
Dec. 31, 2012
Effects of Reinsurance [Line Items]      
Ceded premiums earned $ 440,656,000us-gaap_CededPremiumsEarned $ 455,985,000us-gaap_CededPremiumsEarned $ 396,639,000us-gaap_CededPremiumsEarned
Ceded losses incurred 230,182,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded 188,733,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded 262,649,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
Reserve for uncollectible reinsurance amount 11,300,000us-gaap_ReinsuranceRecoverablesAllowance 11,300,000us-gaap_ReinsuranceRecoverablesAllowance  
Reinsurance rating description To meet our standards of acceptability, when the reinsurance is placed, a reinsurer generally must have a rating from A.M. Best Company ("A.M. Best") and/or S&P of "A" or better, or an equivalent financial strength if not rated, plus at least $500 million in policyholders' surplus.    
Reinsurance recoverables 1,140,696,000us-gaap_ReinsuranceRecoverables [1],[2],[3]    
Collateral Held 201,976,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability    
Percentage of offsetting collateral held 17.70%navg_PercentageOfOffsettingCollateralHeld    
Percentage of reinsurance recoverable of 20 largest reinsurers 75.70%navg_PercentageOfReinsuranceRecoverableForReinsuranceCompanies    
Number of largest reinsurers 20navg_NumberOfLargestReinsurersMeasuredByReinsuranceRecoverable    
Percentage of funding requirement to retain 100.00%navg_PercentageOfFundingRequirement    
Reinsurance recoverables for paid and unpaid losses 902,845,000us-gaap_ReinsuranceReceivablesCaseBasis    
Not rated      
Effects of Reinsurance [Line Items]      
Reinsurance recoverables 5,200,000us-gaap_ReinsuranceRecoverables
/ invest_CreditRatingSourceAxis
= navg_NonRatedMember		    
Percentage of offsetting collateral held 53.60%navg_PercentageOfOffsettingCollateralHeld
/ invest_CreditRatingSourceAxis
= navg_NonRatedMember		    
Ceded Balances Payable      
Effects of Reinsurance [Line Items]      
Collateral Held 152,800,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability
/ us-gaap_CreditFacilityAxis
= navg_CededBalancesPayableMember		    
Letter of Credit      
Effects of Reinsurance [Line Items]      
Collateral Held 43,100,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember		    
Percentage of collateral held 21.00%navg_PercentageOfLettersOfCreditHeldAsCollateral
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember		    
All Other      
Effects of Reinsurance [Line Items]      
Collateral Held 6,100,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability
/ us-gaap_CreditFacilityAxis
= navg_AllOtherMember		    
Catastrophic      
Effects of Reinsurance [Line Items]      
Reinsurance recoverables for paid and unpaid losses 43,300,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= us-gaap_CatastropheMember		 46,500,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= us-gaap_CatastropheMember		  
Superstorm Sandy      
Effects of Reinsurance [Line Items]      
Reinsurance recoverables for paid and unpaid losses 25,400,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= navg_SuperstormSandyMember		 30,700,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= navg_SuperstormSandyMember		  
Hurricane Gustav and Ike      
Effects of Reinsurance [Line Items]      
Reinsurance recoverables for paid and unpaid losses 7,300,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= navg_HurricaneGustavAndIkeMember		 3,300,000us-gaap_ReinsuranceReceivablesCaseBasis
/ us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis
= navg_HurricaneGustavAndIkeMember		  
Minimum      
Effects of Reinsurance [Line Items]      
Policyholders' surplus for reinsurance $ 500,000,000us-gaap_PolicyholdersSurplus
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Percentage of collateral required of qualifying reinsurers 0.00%navg_PercentageOfCollateralRequiredOfQualifyingReinsurers
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum      
Effects of Reinsurance [Line Items]      
Percentage of collateral required of qualifying reinsurers 100.00%navg_PercentageOfCollateralRequiredOfQualifyingReinsurers
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
[1] Net of reserve for uncollectible reinsurance of approximately $11.3 million.
[2] When an A.M. Best rating is unavailable, the equivalent S&P rating is used.
[3] The carrying value is comprised of prepaid reinsurance premium as well as reinsurance recoverables on paid and unpaid losses which are net of the reserve for uncollectible reinsurance.