Currency (Gains) Losses (Included in Other Expense (Income), Net) (Details) - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Currency (gains) losses included in other income expense
Loss (gain) on remeasurement of monetary assets and liabilities, net	$ 30	$ (55)	$ 14
(Gain) loss recognized on balance sheet remeasurement currency exchange contracts, net	0	1	3
Loss released from accumulated other comprehensive loss	(35)	52	10
Currency (gains) losses, net	$ (5)	$ (2)	$ 27

Currency gains losses included in other income expense.

No definition available.

Name:	nvl_CurrencyGainsLossesIncludedInOtherIncomeExpenseAbstract
Namespace Prefix:	nvl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Currency gains (losses) included in other income expense, net.

+ References

No definition available.

+ Details

Name:	nvl_CurrencyGainsLossesIncludedInOtherIncomeExpenseNet
Namespace Prefix:	nvl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Gain (loss) on remeasurement of monetary assets and liabilities net.

+ References

No definition available.

+ Details

Name:	nvl_GainLossOnRemeasurementOfMonetaryAssetsAndLiabilitiesNet
Namespace Prefix:	nvl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of net gains or losses on foreign currency cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

+ References

+ Details

Name:	us-gaap_ForeignCurrencyCashFlowHedgeGainLossReclassifiedToEarningsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of foreign currency derivatives not designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

+ Details

Name:	us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration