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Organization and Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Organization And Summary Of Significant Accounting Policies [Line Items]  
Remeasurement of Foreign Currency $ 10.0us-gaap_ForeignCurrencyTransactionGainBeforeTax
Remeasurement of Foreign currency, After tax $ 6.6navg_ForeignCurrencyTransactionGainAfterTax
Effective Tax Rate 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Syndicate 1221  
Organization And Summary Of Significant Accounting Policies [Line Items]  
Equity Method Investment, Ownership Percentage 100.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ dei_LegalEntityAxis
= navg_SyndicateTwelveHundredTwentyOneMember