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Financial Statement Details - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Net unrealized gains (losses) $ 0nvtl_AvailableforsaleSecuritiesGrossUnrealizedGainLossAccumulatedInAoci $ 5,000nvtl_AvailableforsaleSecuritiesGrossUnrealizedGainLossAccumulatedInAoci  
Property and equipment 0us-gaap_ImpairmentOfLongLivedAssetsHeldForUse 70,000us-gaap_ImpairmentOfLongLivedAssetsHeldForUse 100,000us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
Depreciation and amortization expense $ 6,300,000us-gaap_Depreciation $ 7,900,000us-gaap_Depreciation $ 9,400,000us-gaap_Depreciation