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Fair Value Measurements (Details Textual) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Mar. 31, 2015
Level 3 Instruments [Member]  
Fair Value Measurements [Abstract]  
Unrealized losses related to financial instruments $ 2us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Electricity Swaps [Member]  
Fair Value Measurements [Abstract]  
Average forward price (per megawatt hour) 51nvl_DerivativeForwardPriceEstimatedAveragePrice
/ us-gaap_DerivativeInstrumentRiskAxis
= nvl_ElectricitySwapsMember
Premium over forward prices in nearby observable market (per megawatt hour) 5nvl_DerivativeForwardContractPremiumOverMarketPricePerUnit
/ us-gaap_DerivativeInstrumentRiskAxis
= nvl_ElectricitySwapsMember
Actual swap settlement price (per megawatt hour) 50us-gaap_DerivativeForwardPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= nvl_ElectricitySwapsMember
Change in valuation per a dollar per megawatt hour decline in price (less than) $ 1nvl_DerivativeSensitivityAnalysisChangeInValuationPer1PerMegawattHourChangeInElectricityPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= nvl_ElectricitySwapsMember