Name:	navg_EquitySecuritiesFvNi
Namespace Prefix:	navg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount before tax of accumulated unrealized gain on Equity Securities.

No definition available.

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

Amount before tax of accumulated unrealized loss on Equity Securities.

No definition available.

Name:	navg_EquitySecuritiesFvNiAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	navg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

No definition available.

Name:	navg_EquitySecuritiesFvNiCost
Namespace Prefix:	navg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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.

No definition available.