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Amount and Percentage of Fixed Maturities by Rating (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Investment Securities [Line Items]    
Rating AA  
Fair Value $ 2,365,934us-gaap_AvailableForSaleSecuritiesDebtSecurities $ 2,047,873us-gaap_AvailableForSaleSecuritiesDebtSecurities
Percent of Total 100.00%navg_PercentageOfTotalInvestmentPortfolioFairValue  
Extremely strong    
Investment Securities [Line Items]    
Rating AAA  
Fair Value 462,603us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_ExtremelyStrongMember		  
Percent of Total 20.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_ExtremelyStrongMember		  
Very strong    
Investment Securities [Line Items]    
Rating AA  
Fair Value 1,090,505us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_VeryStrongMember		  
Percent of Total 45.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_VeryStrongMember		  
Strong    
Investment Securities [Line Items]    
Rating A  
Fair Value 612,131us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_StrongMember		  
Percent of Total 26.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_StrongMember		  
Adequate    
Investment Securities [Line Items]    
Rating BBB  
Fair Value 185,372us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_AdequateMember		  
Percent of Total 8.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_AdequateMember		  
Speculative    
Investment Securities [Line Items]    
Rating BB & Below  
Fair Value 14,852us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_SpeculativeMember		  
Percent of Total 1.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_SpeculativeMember		  
Not rated    
Investment Securities [Line Items]    
Rating NR  
Fair Value $ 471us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ navg_CreditRatingAxis
= navg_NonRatedMember		  
Percent of Total 0.00%navg_PercentageOfTotalInvestmentPortfolioFairValue
/ navg_CreditRatingAxis
= navg_NonRatedMember