Additional Financial Data by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Gross written premiums	$ 396,460us-gaap_PremiumsWrittenGross	$ 422,790us-gaap_PremiumsWrittenGross
Ceded written premiums	(107,502)us-gaap_CededPremiumsWritten	(110,940)us-gaap_CededPremiumsWritten
Net written premiums	288,958us-gaap_PremiumsWrittenNet	311,850us-gaap_PremiumsWrittenNet
Net earned premiums	236,132us-gaap_PremiumsEarnedNet	225,272us-gaap_PremiumsEarnedNet
% Change Gross written premiums	(6.20%)navg_PercentageOfGrossWrittenPremium
% Change Ceded written premiums	(3.10%)navg_PercentageOfCededPremiums
% Change Net written premiums	(7.30%)navg_PercentageOfNetWrittenPremium
% Change Net earned premiums	4.80%navg_PercentageOfNetEarnedPremium
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums	199,327	198,348
Ceded written premiums	(68,730)	(65,066)
Net written premiums	130,597	133,282
Net earned premiums	131,091	115,190
% Change Gross written premiums	0.50%
% Change Ceded written premiums	5.60%
% Change Net written premiums	(2.00%)
% Change Net earned premiums	13.80%
U.S.Insurance \| Marine
Segment Reporting Information [Line Items]
Gross written premiums	40,835	43,025
Ceded written premiums	(16,286)	(9,170)
Net written premiums	24,549	33,855
Net earned premiums	23,924	27,986
% Change Gross written premiums	(5.10%)
% Change Ceded written premiums	77.60%
% Change Net written premiums	(27.50%)
% Change Net earned premiums	(14.50%)
U.S.Insurance \| Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	134,146	125,119
Ceded written premiums	(39,558)	(46,156)
Net written premiums	94,588	78,963
Net earned premiums	90,735	64,931
% Change Gross written premiums	7.20%
% Change Ceded written premiums	(14.30%)
% Change Net written premiums	19.80%
% Change Net earned premiums	39.70%
U.S.Insurance \| Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	24,346	30,204
Ceded written premiums	(12,886)	(9,740)
Net written premiums	11,460	20,464
Net earned premiums	16,432	22,273
% Change Gross written premiums	(19.40%)
% Change Ceded written premiums	32.30%
% Change Net written premiums	(44.00%)
% Change Net earned premiums	(26.20%)
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums	122,427	119,192
Ceded written premiums	(34,256)	(42,582)
Net written premiums	88,171	76,610
Net earned premiums	65,525	62,054
% Change Gross written premiums	2.70%
% Change Ceded written premiums	(19.60%)
% Change Net written premiums	15.10%
% Change Net earned premiums	5.60%
International Insurance \| Marine
Segment Reporting Information [Line Items]
Gross written premiums	67,410	61,982
Ceded written premiums	(10,822)	(12,394)
Net written premiums	56,588	49,588
Net earned premiums	37,792	38,044
% Change Gross written premiums	8.80%
% Change Ceded written premiums	(12.70%)
% Change Net written premiums	14.10%
% Change Net earned premiums	(0.70%)
International Insurance \| Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums	32,950	42,250
Ceded written premiums	(16,129)	(24,464)
Net written premiums	16,821	17,786
Net earned premiums	16,034	16,441
% Change Gross written premiums	(22.00%)
% Change Ceded written premiums	(34.10%)
% Change Net written premiums	(5.40%)
% Change Net earned premiums	(2.50%)
International Insurance \| Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums	22,067	14,960
Ceded written premiums	(7,305)	(5,724)
Net written premiums	14,762	9,236
Net earned premiums	11,699	7,569
% Change Gross written premiums	47.50%
% Change Ceded written premiums	27.60%
% Change Net written premiums	59.80%
% Change Net earned premiums	54.60%
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	74,706	105,250
Ceded written premiums	(4,516)	(3,292)
Net written premiums	70,190	101,958
Net earned premiums	$ 39,516	$ 48,028
% Change Gross written premiums	(29.00%)
% Change Ceded written premiums	37.20%
% Change Net written premiums	(31.20%)
% Change Net earned premiums	(17.70%)

Percentage of Ceded Premiums

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- Definition

Percentage of Gross Written Premium

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- Definition

Percentage of Net Earned Premium

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Percentage of Net Written Premium

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- Definition

Amount of written premiums ceded to other entities.

+ References

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(c))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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- Definition

Amount of direct and assumed premiums written.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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