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Amount and Nature of Net Assets Restricted from Payment of Dividends (Parenthetical) (Detail) (Fixed maturities, USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Insurance Companies    
Restricted Assets And Liabilities Relating To Securitization [Line Items]    
Restricted net assets, amortized cost $ 10,086navg_FixedMaturitySecuritiesAmortizedCost
/ navg_RestrictedAssetsAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_InsuranceCompaniesMember		 $ 9,730navg_FixedMaturitySecuritiesAmortizedCost
/ navg_RestrictedAssetsAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_InsuranceCompaniesMember
Lloyd's    
Restricted Assets And Liabilities Relating To Securitization [Line Items]    
Restricted net assets, amortized cost $ 445,504navg_FixedMaturitySecuritiesAmortizedCost
/ navg_RestrictedAssetsAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember		 $ 398,930navg_FixedMaturitySecuritiesAmortizedCost
/ navg_RestrictedAssetsAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember