Restricted Stock Unit Awards (Parenthetical) (Detail) - Restricted Stock Units (RSUs)	6 Months Ended
Jun. 30, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards withheld for tax payments \| shares	25,814
Weighted average vest date fair value, withheld for tax payments \| $ / shares	$ 55.21

Share based compensation arrangement by Share based payment award equity instruments other than options withheld for tax payment in period weighted average vest date fair value.

No definition available.

Name:	navg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWithheldForTaxPaymentInPeriodWeightedAverageVestDateFairValue
Namespace Prefix:	navg_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration