This item contains disclosure of the number of investment positions in the available-for-sale investments that have been determined to be other than temporarily impaired (both credit and non-credit).

No definition available.

+ Details

Name:	navg_NumberOfSecuritiesOtherThanTemporaryImpairmentsLosses
Namespace Prefix:	navg_
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Balance Type:	na
Period Type:	duration

- 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=31042434&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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount after tax of other than temporary impairment (OTTI) loss on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 35

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25362-111560

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 10A

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580

+ Details

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

- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 8B

-URI http://asc.fasb.org/extlink&oid=27724398&loc=SL6284393-111563

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Period Type:	duration

Summary of Activity Related to Other-Than-Temporary Impairment Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013 Investment
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities		2
Total other-than-temporary impairment losses		$ 42
Portion of loss recognized in other comprehensive income
Net other-than-temporary impairment losses recognized in earnings		42
Corporate And Other Bonds
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Portion of loss recognized in other comprehensive income
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Portion of loss recognized in other comprehensive income
Residential Mortgage Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Portion of loss recognized in other comprehensive income
Asset-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Portion of loss recognized in other comprehensive income
Equity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Number of Securities		2
Total other-than-temporary impairment losses		42
Portion of loss recognized in other comprehensive income
Net other-than-temporary impairment losses recognized in earnings		$ 42