Date when the business acquisition agreement was executed, in CCYY-MM-DD format.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
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Period Type:	duration

- 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

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Data Type:	xbrli:stringItemType
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- Definition

Name of the acquired entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

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- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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Balance Type:	na
Period Type:	instant

- Definition

Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

The amount of indemnification assets (amounts to be reimbursed if and when certain assumed liabilities are paid) recognized at the acquisition date of a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

+ Details

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

- Definition

For indemnification assets recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the indemnification benefit which may be realized.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472

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- Definition

This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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Balance Type:	credit
Period Type:	duration

- Definition

This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to intangibles acquired in connection with a business combination for which the initial accounting was incomplete.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 4A
-Subparagraph (c)
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Balance Type:	debit
Period Type:	duration

- Definition

Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

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-Publisher FASB
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-SubTopic 20
-Section 50
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+ Details

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- Definition

The amount of identifiable intangible assets recognized as of the acquisition date.

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- Definition

Weighted average period before the next renewal or extension (both explicit and implicit) for intangible assets that have been renewed or extended, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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-SubTopic 30
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-Paragraph 2
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Period Type:	duration

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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+ Details

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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
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- Details

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- Details

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Name:	us-gaap_BusinessAcquisitionAxis=navg_BrachtDeckersMackelbertNVAndAssurancesContinentalesContinentaleVerzekeringenNVMember
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Merger And Business Combinations - Additional Information (Detail) $ / shares in Units, € in Millions				6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 07, 2018 USD ($)	Jun. 07, 2018 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Aug. 22, 2018 $ / shares	Jun. 07, 2018 EUR (€)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill recognized at date of acquisition				$ 16,818,000	$ 16,818,000			$ 4,460,000
Finite-Lived Intangible Assets
Business Acquisition [Line Items]
Increase (decrease) in fair value of identifiable intangible assets and goodwill	$ (1,400,000)
Goodwill
Business Acquisition [Line Items]
Increase (decrease) in fair value of identifiable intangible assets and goodwill	$ 1,400,000
The Hartford
Business Acquisition [Line Items]
Merger Consideration \| $ / shares						$ 70.00
Bracht, Deckers & Mackelbert NV and Assurances Continentales - Continentale Verzekeringen NV
Business Acquisition [Line Items]
Acquisition date		Jun. 07, 2018	Jun. 07, 2018
Business acquisition, name of acquired entity					BDM, ASCO, and a wholly-owned subsidiary of ASCO, Canal Re S.A.
Business acquisition, percentage of ownership interest acquired		100.00%					100.00%
Business acquisition, purchase price		$ 40,492,000	€ 35.0
Additional reimbursement amount receivable		5,800,000					€ 5.0
Business combination, fair value of acquisition reimbursement amount		0		0	$ 0
Goodwill recognized at date of acquisition		12,703,000
Business combination, identified finite lived intangible assets		$ 6,500,000
Business combination, finite lived intangible assets weighted average amortization period		9 years	9 years
Business combination, indefinite intangible assets		$ 2,500,000
Revenues				11,100,000
Net Income (Loss)				$ (300,000)