Other Than Temporary Impairment Non Credit Losses Portion Net Unrealized Losses Gains Related To Securities Net of Tax

No definition available.

Name:	navg_OtherThanTemporaryImpairmentNonCreditLossesPortionNetUnrealizedLossesGainsRelatedToSecuritiesNetOfTax
Namespace Prefix:	navg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of fair value under carrying value.

+ References

No definition available.

+ Details

Name:	navg_PercentageOfFairValueUnderCarryingValue
Namespace Prefix:	navg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted Average Expected Maturity Of Securities

+ References

No definition available.

+ Details

Name:	navg_WeightedAverageExpectedMaturityOfSecurities
Namespace Prefix:	navg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

+ Details

Name:	us-gaap_ImpairmentOfInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percent of net assets at close of period. For schedules of investments that are categorized, each category has a percent of net assets for the aggregated value of the Investments in the category.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-12.3)

-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 4

-Subparagraph (SX 210.12-12C.Column D.2)

-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010

+ Details

Name:	us-gaap_InvestmentOwnedPercentOfNetAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=navg_MortgageAndAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=navg_NonGovernmentBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments - Additional Information (Detail) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) EquityPlan Investment	Jun. 30, 2015 USD ($) EquityPlan Investment	Dec. 31, 2014 USD ($) EquityPlan Investment	Jun. 30, 2014 USD ($)
Schedule of Investments [Line Items]
Non-credit OTTI on fixed maturities in an unrealized gain position		$ 600	$ 700
Gross Unrealized Loss	$ 19,658	19,658	$ 11,651
OTTI losses	423	423
Credit loss portion of the OTTI losses on fixed maturities	$ 2,400	$ 2,400		$ 5,200
Equity Securities
Schedule of Investments [Line Items]
Number of unrealized loss position \| EquityPlan	49	49	15
Gross Unrealized Loss	$ 3,572	$ 3,572	$ 1,304
Equity Securities \| Maximum
Schedule of Investments [Line Items]
Percentage of fair value under carrying value	80.00%	80.00%
Fixed maturities
Schedule of Investments [Line Items]
Number of unrealized loss position \| Investment	345	345	259
Gross Unrealized Loss	$ 16,086	$ 16,086	$ 10,347
Mortgage-backed and asset-backed securities
Schedule of Investments [Line Items]
Securities, effective maturities		4 years 8 months 12 days
Mortgage-backed and asset-backed securities \| Fixed maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	4,171	$ 4,171	2,198
Non-government-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 900	$ 900	$ 500
Non-government-backed securities \| Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%	5.00%	5.00%