Significant Accounting Policies - Schedule of Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2025
Regulatory Liabilities [Line Items]
Regulatory liability, current	$ 116,034	$ 137,974	$ 130,944
Regulatory liability, noncurrent	779,458	758,407	737,194
SiEnergy
Regulatory Liabilities [Line Items]
Regulatory liability, current	62	66	24
Regulatory liability, noncurrent	9,114	8,155	0
NWN Water
Regulatory Liabilities [Line Items]
Regulatory liability, current	101	0	148
Regulatory liability, noncurrent	1,303	1,406	943
NW Natural
Regulatory Liabilities [Line Items]
Regulatory liability, current	115,871	137,908	130,772
Regulatory liability, noncurrent	769,041	748,846	736,251
NW Natural \| Gas costs
Regulatory Liabilities [Line Items]
Regulatory liability, current	17,217	34,172	41,049
Regulatory liability, noncurrent	1,083	687	5,133
NW Natural \| Deferred Derivative Gain (Loss)
Regulatory Liabilities [Line Items]
Regulatory liability, current	4,338	3,922	7,182
Regulatory liability, noncurrent	3,430	366	4,077
NW Natural \| Decoupling
Regulatory Liabilities [Line Items]
Regulatory liability, current	1,691	5,181	6,811
Regulatory liability, noncurrent	0	0	841
NW Natural \| Income taxes
Regulatory Liabilities [Line Items]
Regulatory liability, current	5,406	5,406	4,726
Regulatory liability, noncurrent	154,961	159,176	160,990
NW Natural \| Asset optimization revenue sharing
Regulatory Liabilities [Line Items]
Regulatory liability, current	13,971	25,981	14,121
Regulatory liability, noncurrent	0	2,386	0
NW Natural \| Washington Climate Commitment Act compliance
Regulatory Liabilities [Line Items]
Regulatory liability, current	66,556	57,441	51,801
Regulatory liability, noncurrent	5,000	0	0
NW Natural \| Accrued asset removal costs
Regulatory Liabilities [Line Items]
Regulatory liability, noncurrent	580,531	561,594	543,700
NW Natural \| Other
Regulatory Liabilities [Line Items]
Regulatory liability, current	6,692	5,805	5,082
Regulatory liability, noncurrent	$ 24,036	$ 24,637	$ 21,510

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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