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Credit Quality Distribution of Reinsurance Recoverables for Ceded Paid and Unpaid Losses and Loss Adjustment Expenses and Ceded Unearned Premiums Based On Insurer Financial Strength Ratings from A. M. Best or S&P (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Ceded Credit Risk [Line Items]  
Rating AA
Fair Value $ 1,140,696us-gaap_ReinsuranceRecoverables [1],[2],[3]
Percent of Total 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal [1]
Superior  
Ceded Credit Risk [Line Items]  
Rating A++, A+ [1]
Fair Value 575,205us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= navg_SuperiorMember		 [1],[3]
Percent of Total 50.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= navg_SuperiorMember		 [1]
Excellent  
Ceded Credit Risk [Line Items]  
Rating A, A- [1]
Fair Value 547,314us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= navg_ExcellentMember		 [1],[3]
Percent of Total 48.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= navg_ExcellentMember		 [1]
Very Good  
Ceded Credit Risk [Line Items]  
Rating B++, B+ [1]
Fair Value 8,483us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= navg_VeryGoodMember		 [1],[3]
Percent of Total 1.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= navg_VeryGoodMember		 [1]
Fair  
Ceded Credit Risk [Line Items]  
Rating B, B- [1]
Percent of Total 0.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= navg_FairMember		 [1]
Not rated  
Ceded Credit Risk [Line Items]  
Rating NR [1]
Fair Value $ 9,695us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= navg_NonRatedMember		 [1],[3]
Percent of Total 1.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= navg_NonRatedMember		 [1]
[1] When an A.M. Best rating is unavailable, the equivalent S&P rating is used.
[2] Net of reserve for uncollectible reinsurance of approximately $11.3 million.
[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.