Performance-based Equity Awards (Parenthetical) (Detail) - Performance-based Equity Awards	9 Months Ended
Sep. 30, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share awards withheld for tax payments \| shares	117,005
Weighted average vest date fair value, withheld for tax payments \| $ / shares	$ 55.65

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.

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