Environmental Matters (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)
Accrual for Environmental Loss Contingencies [Abstract]
Site Contingency, Number of Remediation Projects			2
Environmental Regulatory Table [Abstract]
Regulatory Assets, Current	$ 41,929,000		$ 31,745,000
Regulatory Assets, Noncurrent	343,146,000		$ 348,927,000
Prudent Costs Deferred During Period	3,000,000.0	$ 1,500,000
Insurance Proceeds, Amortization Period			10 years 6 months
Cost Recovery Period			3 years
WASHINGTON
Environmental Activity [Line Items]
Remediation Recovery Percentage			3.30%
Northwest Natural Gas Company [Member]
Environmental Activity [Line Items]
Remediation Recovery Percentage			96.70%
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	39,225,000		$ 28,241,000
Environmental Noncurrent Liabilities	96,797,000		92,281,000
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			1,050,000,000.00
Annual Tariff Rider Collection			5,000,000.0
Environmental Settlement Insurance Proceeds Received To Date			150,000,000
Annual insurance proceeds to appy against remediation costs			5,000,000.0
Insurance Proceeds Applied To Prudently Incurred Remediation Costs			83,200,000
Environmental Regulatory Table [Abstract]
Regulatory Assets, Current	41,929,000		31,745,000
Regulatory Assets, Noncurrent	343,146,000		348,887,000
Annual Insurance Proceeds to Apply Against Remediation Costs			5,000,000.0
Total Annual Remediation Expense and Interest			10,000,000.0
Northwest Natural Gas Company [Member] | Minimum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			(0.30)
Northwest Natural Gas Company [Member] | Maximum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
EPA Record Of Decision, Clean-Up Costs, Portland Harbor			0.50
Northwest Natural Gas Company [Member] | Gasco Siltronic Sediments [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	11,632,000		7,596,000
Environmental Noncurrent Liabilities	46,082,000		43,725,000
Northwest Natural Gas Company [Member] | Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	2,543,000		1,942,000
Environmental Noncurrent Liabilities	6,920,000		7,020,000
Northwest Natural Gas Company [Member] | Gasco Upland [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	14,203,000		14,887,000
Environmental Noncurrent Liabilities	43,616,000		40,250,000
Northwest Natural Gas Company [Member] | Central Service Center [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	0		0
Environmental Noncurrent Liabilities	0		0
Northwest Natural Gas Company [Member] | Front Street [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	10,847,000		3,816,000
Environmental Noncurrent Liabilities	0		1,107,000
Northwest Natural Gas Company [Member] | Oregon Steel Mills [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Current Liabilities	0		0
Environmental Noncurrent Liabilities	179,000		179,000
Northwest Natural Gas Company [Member] | Environmental Restoration Costs [Member]
Environmental Regulatory Table [Abstract]
Environmental Regulatory Deferred Cost and Interest	36,673,000		44,516,000
Environmental Regulatory Accrued Site Liabilities	135,662,000		120,352,000
Environmental Regulatory Insurance Proceeds and Interest	(79,949,000)		(69,253,000)
Regulatory Assets, Current	4,762,000		4,992,000
Regulatory Assets, Noncurrent	87,624,000		90,623,000
Regulatory Assets	92,386,000		95,615,000
Northwest Natural Gas Company [Member] | Environmental Restoration Costs [Member] | Front Street [Member]
Environmental Regulatory Table [Abstract]
Environmental Regulatory Accrued Site Liabilities	$ 400,000		200,000
Front Street [Member] | Northwest Natural Gas Company [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Undiscounted Environmental Remediation Expense			7,100,000
Liability For Studies And Design Costs			$ 4,900,000
Portland Harbor [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Number Of Potentially Responsible Parties			one hundred
Gasco Siltronic Sediments [Member] | Minimum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Exit Costs, Anticipated Cost			$ 51,300,000
Gasco Siltronic Sediments [Member] | Maximum [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Exit Costs, Anticipated Cost			350,000,000
Gasco Siltronic Sediments [Member] | Northwest Natural Gas Company [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Environmental Project Liability			$ 51,300,000
Yakama Nation [Member] | Portland Harbor Other [Member]
Accrual for Environmental Loss Contingencies [Abstract]
Trustee Council Members - Number of Claimants			1
Site Contingency, Number of Other Third-Party Defendants			29
NRD Liability Claim Yakama Nation			$ 300,000
Number of Amended Complaints Filed			2