- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

TargetHedgeAchieved

+ References

No definition available.

+ Details

Name:	nwn_TargetHedgeAchieved
Namespace Prefix:	nwn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nwn_FinancialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nwn_GasYearAxis=nwn_GasYear20162017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nwn_GasYearAxis=nwn_GasYear20172018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=nwn_PhysicalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=nwn_NaturalGasThermsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Instruments Notional Amounts (Details) therm in Thousands, $ in Thousands	Sep. 30, 2018 USD ($) therm	Dec. 31, 2017 USD ($) therm	Sep. 30, 2017 USD ($) therm
Financial [Member] \| Gas Year 2016 - 2017
Derivative [Line Items]
TargetHedgeAchieved	48.00%
Financial [Member] \| Gas Year 2017 - 2018 [Member]
Derivative [Line Items]
TargetHedgeAchieved	49.00%
Physical [Member] \| Gas Year 2016 - 2017
Derivative [Line Items]
TargetHedgeAchieved	27.00%
Physical [Member] \| Gas Year 2017 - 2018 [Member]
Derivative [Line Items]
TargetHedgeAchieved	26.00%
Natural Gas Therms [Member] \| Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	513,850	429,100	521,080
Natural Gas Therms [Member] \| Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	760,925	520,268	750,650
Foreign Exchange Contract [Member]
Derivative [Line Items]
Foreign exchange \| $	$ 7,184	$ 7,669	$ 6,933