Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Abstract]
Short-term Debt	$ 53,300	$ 270,035
Commercial Paper	53,300	220,000
Bank Loans	$ 0	$ 50,000
Short-term Debt, Weighted Average Interest Rate	0.80%	0.60%
Commercial Paper, Maximum Maturity	11 days
Commercial Paper, Average Maturity	6 days
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 97,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	30,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	75,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	60,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	424,700
Debt Outstanding
Long-term Debt	719,323	$ 594,418
Long-term Debt, Current Maturities	39,989	24,973
Long-term debt, noncurrent	679,334	569,445
Repayments of Long-term Debt [Abstract]
Long-term Debt, Gross	726,700	601,700
Unamortized Debt Issuance Expense	(7,377)	(7,282)
Long-Term Debt, Current Maturities, Gross	40,000	25,000
Long-term Debt, Excluding Maturities, Gross	686,700	576,700
Liabilities, Fair Value Disclosure [Abstract]
Debt Instrument, Fair Value Disclosure	793,339	667,168
Note 5150 Series B Due 2016 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 0	$ 25,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	0.00%	5.15%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 25,000
Note 7000 Series B Due 2017 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Note 1545 Series Due 2018 [Domain]
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 75,000
Secured Debt	$ 75,000	$ 0
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	1.545%	0.00%
Note 6600 Series B Due 2018 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 22,000	$ 22,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.60%	6.60%
Note 8310 Series B Due 2019 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	8.31%	8.31%
Note 7630 Series B Due 2019 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.63%	7.63%
Note 5370 Series B Due 2020 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 75,000	$ 75,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.37%	5.37%
Note 9050 Series A Due 2021 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	9.05%	9.05%
Note 3176 Series B Due 2021 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.176%	3.176%
Note 3542 Series B Due 2023 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.542%	3.542%
Note 5620 Series B Due 2023 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.62%	5.62%
Note 7720 Series B Due 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.72%	7.72%
Note 6520 Series B Due 2025 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.52%	6.52%
Note 7050 Series B Due 2026 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.05%	7.05%
Note 3211 Series B Due 2026 [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 35,000
Secured Debt	$ 35,000	$ 0
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	3.211%	0.00%
Note 7000 Series Due 2027 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Note 6650 Series Due 2027 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 19,700	$ 19,700
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 6650 Series Due 2028 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 7740 Series Due 2030 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 20,000	$ 20,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.74%	7.74%
Note 7850 Series Due 2030 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	7.85%	7.85%
Note 5820 Series Due 2032 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 30,000	$ 30,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.82%	5.82%
Note 5660 Series Due 2032 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 40,000	$ 40,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.66%	5.66%
Note 5250 Series Due 2035 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,000	$ 10,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	5.25%	5.25%
Note 4136 Series Due 2046 [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 40,000
Secured Debt	$ 40,000	$ 0
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.136%	0.00%
Note 4000 Series Due 2042 [Member]
Debt Instrument [Line Items]
Secured Debt	$ 50,000	$ 50,000
Debt Outstanding
Debt Instrument, Interest Rate, Stated Percentage	4.00%	4.00%
Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 300,000
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity	$ 450,000
Line of Credit Facility, Expiration Date	Dec. 20, 2019
Line of Credit Facility, Amount Outstanding	$ 0	$ 0
Debt Instrument Covenant Consolidated Indebtedness To Capitalization Ratio	70.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000
Letters of Credit Outstanding, Amount	$ 0	$ 0

Commercial Paper, Average Maturity (duration)

No definition available.

Name:	nwn_CommercialPaperAverageMaturity
Namespace Prefix:	nwn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Commercial Paper, Maximum Maturity (duration)

+ References

No definition available.

+ Details

Name:	nwn_CommercialPaperMaximumMaturity
Namespace Prefix:	nwn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum ratio of consolidated indebtedness to total capitalizationwhich is necessary to be maintained under the terms of the line of credit facility.

+ References

No definition available.

+ Details

Name:	nwn_DebtInstrumentCovenantConsolidatedIndebtednessToCapitalizationRatio
Namespace Prefix:	nwn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increased maximum borrowing capacity under the credit facility which the reporting entity may request.

+ References

No definition available.

+ Details

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

- Definition

Long-Term Debt, Current Maturities, Gross

+ References

No definition available.

+ Details

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

- Definition

Long-term Debt, Excluding Maturities, Gross

+ References

No definition available.

+ Details

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

- Definition

Amounts due on short-term bank loans secured by customers' securities on the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_BankLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_CommercialPaper
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13476-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

Name:	us-gaap_ExtinguishmentOfDebtAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

Name:	us-gaap_LongTermDebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

Name:	us-gaap_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

No definition available.

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_SecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Weighted average interest rate of short-term debt outstanding.

+ References

+ Details

Name:	us-gaap_ShortTermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5150SeriesDue2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7000SeriesDue2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note1545SeriesDue2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note6600SeriesDue2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note8310SeriesDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7630SeriesDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5370SeriesDue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note9050SeriesDue2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note3176SeriesDue2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note3542SeriesDue2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5620SeriesDue2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7720SeriesDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note6520SeriesDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7050SeriesDue2026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note3211SeriesBDue2026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7000SeriesDue2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note6650SeriesDue2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note6650SeriesDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7740SeriesDue2030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note7850SeriesDue2030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5820SeriesDue2032Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5660SeriesDue2032Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note5250SeriesDue2035Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note4136SeriesDue2046Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nwn_Note4000SeriesDue2042Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: