Name:	nwn_GasReservesAbstract
Namespace Prefix:	nwn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Cumulative regulatory amortization of gas reserves investment.

No definition available.

Name:	nwn_GasReservesAmortization
Namespace Prefix:	nwn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Gross carrying amount as of the balance sheet date of investments in Gas Reserves expected to be recovered in longer than one year.

No definition available.

Name:	nwn_GasReservesNoncurrentGross
Namespace Prefix:	nwn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Gas reserves investment net of amortization and deferred taxes.

No definition available.

Name:	nwn_NetGasReservesInvestment
Namespace Prefix:	nwn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Total gas reserves net of amortization.

No definition available.

Name:	nwn_TotalGasReserves
Namespace Prefix:	nwn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Carrying amount as of the balance sheet date of capitalized gas costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378