Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Ceded Credit Risk [Line Items]
Reserve for uncollectible reinsurance amount	$ 13.0

A valuation allowance for the amount of reinsurance recoverable deemed uncollectible.

No definition available.

Name:	navg_ReinsuranceUncollectibleAccounts
Namespace Prefix:	navg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant