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Credit Facilities - Additional Information (Detail)
In Millions, unless otherwise specified
12 Months Ended 12 Months Ended 0 Months Ended 0 Months Ended
Dec. 31, 2014
Mar. 28, 2011
USD ($)
Dec. 31, 2014
Line of Credit
USD ($)
Dec. 31, 2014
Syndicate 1221
Nov. 06, 2014
Syndicate 1221
AUD
Nov. 06, 2014
Syndicate 1221
GBP (£)
Nov. 06, 2014
Syndicate 1221
Above Floating Rate
Line of Credit
Dec. 31, 2014
Syndicate 1221
Lloyd's
USD ($)
Nov. 22, 2012
Amended and Restated Credit Agreement
Nov. 22, 2012
Amended and Restated Credit Agreement
USD ($)
Line of Credit Facility [Line Items]                    
Line of credit facility, maximum borrowing amount         8.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember
         
Line of credit facility, underwriting reserve required           75navg_LineOfCreditFacilityCovenantRequiredUnderwritingReserve
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember
       
Line of credit facility, covenant compliance The Company was in compliance with all covenants under the credit facility as of December 31, 2014 and the Company had $1.0 million of cash collateral posted.     As of December 31, 2014, our Company was in compliance with all covenants.            
Line of credit facility, interest rate             2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember
/ us-gaap_VariableRateAxis
= navg_FloatingRateMember
     
Line of credit facility   175us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity               165us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= navg_AmendedAndRestatedCreditAgreementMember
Issuance of new letters of credit, expiry date                 Dec. 31, 2016  
Aggregate face amount of letters of credit outstanding               149.4us-gaap_LineOfCredit
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember
   
Cash collateral posted     $ 1.0us-gaap_CashCollateralForBorrowedSecurities
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
        $ 1.0us-gaap_CashCollateralForBorrowedSecurities
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember
/ us-gaap_StatementBusinessSegmentsAxis
= navg_LloydOperationsMember
   
Line of credit facility, covenant terms This credit facility contains customary covenants for facilities of this type, including restrictions on indebtedness and liens, limitations on mergers, dividends and the sale of assets, and requirements as to maintaining certain consolidated tangible net worth, statutory surplus and other financial ratios. The credit facility also provides for customary events of default, including failure to pay principal, interest or fees when due, failure to comply with covenants, any representation or warranty made by the Company being false in any material respect, default under certain other indebtedness, certain insolvency or receivership events affecting the Company and its subsidiaries, the occurrence of certain material judgments, or a change in control of the Company.