Name:	ng_UnconsolidatedVenturesNumberOfDirectors
Namespace Prefix:	ng_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of directors on the board of unconsolidated ventures, which directors are selected by the reporting entity.

No definition available.

Name:	ng_UnconsolidatedVenturesNumberOfDirectorsSelectedByTheReportingEntity
Namespace Prefix:	ng_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of directors on the board of unconsolidated ventures, which directors are selected by the reporting entity's venture partners.

No definition available.

Name:	ng_UnconsolidatedVenturesNumberOfDirectorsSelectedByVenturePartners
Namespace Prefix:	ng_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Name:	us-gaap_EquityMethodInvestmentAggregateCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481687/323-10-50-3