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Senior Notes - Additional Information (Detail) (USD $)
 3 Months Ended 12 Months Ended 0 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Oct. 04, 2013
Debt Instrument [Line Items]                        
Interest rate stated on senior notes 7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage               7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage      
Senior notes due date                 May 01, 2016      
Call premium on redemption of Senior Notes         $ (17,895,000)us-gaap_GainsLossesOnExtinguishmentOfDebt         $ (17,895,000)us-gaap_GainsLossesOnExtinguishmentOfDebt    
Interest expense 3,854,000us-gaap_InterestExpense 3,388,000us-gaap_InterestExpense 4,319,000us-gaap_InterestExpense 3,852,000us-gaap_InterestExpense 4,351,000us-gaap_InterestExpense 2,053,000us-gaap_InterestExpense 2,052,000us-gaap_InterestExpense 2,051,000us-gaap_InterestExpense 15,413,000us-gaap_InterestExpense 10,507,000us-gaap_InterestExpense 8,198,000us-gaap_InterestExpense  
Senior Notes                        
Debt Instrument [Line Items]                        
Unamortized discount 1,600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       1,700,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       1,600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 1,700,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		    
Interest expense                   10,500,000us-gaap_InterestExpense
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		    
Senior Notes | 5.75% Senior Notes                        
Debt Instrument [Line Items]                        
Principal amount of senior notes                       265,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
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= us-gaap_SeniorNotesMember
Interest rate stated on senior notes                       5.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Senior notes due date                 Oct. 15, 2023      
Net proceeds from issuance of senior notes                       263,000,000us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt
/ us-gaap_DebtInstrumentAxis
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= us-gaap_SeniorNotesMember
Outstanding principal amount of senior notes 263,400,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       263,300,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       263,400,000us-gaap_DebtInstrumentCarryingAmount
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= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 263,300,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		    
Fair value of senior notes 285,700,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       277,600,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		       285,700,000us-gaap_DebtInstrumentFairValue
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= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 277,600,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		    
Effective interest rate related to senior notes 5.86%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		               5.86%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		      
Interest expense                 15,400,000us-gaap_InterestExpense
/ us-gaap_DebtInstrumentAxis
= navg_SeniorNotesFivePointSevenFivePercentageMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		      
Debt instrument, covenant compliance                 As of December 31, 2014, our Company was in compliance with all such covenants.      
Senior Notes | 5.75% Senior Notes | Semi Annual Payment, First Payment                        
Debt Instrument [Line Items]                        
Interest payment date on senior notes                 --04-15      
Senior Notes | 5.75% Senior Notes | Semi Annual Payment, Second Payment                        
Debt Instrument [Line Items]                        
Interest payment date on senior notes                 --10-15      
Senior Notes | 7.0% Senior Notes                        
Debt Instrument [Line Items]                        
Interest rate stated on senior notes 7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= navg_SevenPercentageSeniorNotesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		               7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
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= us-gaap_SeniorNotesMember		      
Senior notes due date                 May 01, 2016      
Call premium on redemption of Senior Notes                       17,900,000us-gaap_GainsLossesOnExtinguishmentOfDebt
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Interest expense on senior notes                     $ 8,200,000us-gaap_InterestExpenseLongTermDebt
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