Recent Accounting Pronouncements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2012
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in capitalized acquisition costs resulting from adoption of ASU 2010-26	$ 1.2	$ 2.9

Impact of the prospective adoption of the revised accounting guidance for the deferral of policy acquisition costs.

No definition available.

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