Income Tax (Details) - USD ($)		3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				$ 0	$ (76,000)	$ 0
Regulatory Settlement				(1,129,000)	0	0
Deferred Tax Liabilities, Property, Plant and Equipment		$ 269,886,000		269,886,000	288,385,000
Deferred Tax Liabilities, Leasing Arrangements		40,133,000		40,133,000	0
Deferred Tax Liabilities Non Regulated Liabilities		751,000		751,000	1,061,000
Deferred Tax Liabilities, Net		295,643,000		295,643,000	280,463,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax		100,000		100,000
Deferred Tax Assets, Tax Credit Carryforwards, Other		162,000		162,000	52,000
Deferred Tax Assets, Net		57,144,000		57,144,000	57,521,000
Federal Statutory Income Tax Rate, Percent	21.00%						35.00%
Increase (Decrease) in Regulatory Liabilities				853,000	7,929,000	0
Effect of TCJA				0	0	(3,376,000)
Regulatory Liability, Noncurrent		625,717,000		625,717,000	611,560,000
Unrecognized Tax Benefits	$ 0	0		0	0	0
Deferred Tax Assets, Goodwill and Intangible Assets		2,723,000		2,723,000	0
Deferred Tax Liabilities, Gross		352,787,000		352,787,000	337,984,000
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				16,370,000	19,222,000	39,578,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,422,000	4,927,000	5,066,000
Income Tax Reconciliation, Differences Regulatory Commission				(5,772,000)	1,302,000	2,357,000
Effect of TCJA				0	0	(3,376,000)
Income Tax Reconciliation, Other Adjustments				(1,249,000)	(1,184,000)	(2,617,000)
Income Tax Expense (Benefit)				$ 12,642,000	$ 24,191,000	$ 41,008,000
Effective Income Tax Rate				16.20%	26.40%	36.30%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 5,530,000	$ 8,953,000	$ 19,345,000
Current State and Local Tax Expense (Benefit)				1,667,000	3,785,000	5,963,000
Current Income Tax Expense (Benefit)				7,197,000	12,738,000	25,308,000
Deferred Federal Income Tax Expense (Benefit)				1,515,000	9,001,000	13,869,000
Deferred State and Local Income Tax Expense (Benefit)				3,930,000	2,452,000	1,831,000
Deferred Income Tax Expense (Benefit)				5,445,000	11,453,000	15,700,000
Income Tax Expense (Benefit)				12,642,000	24,191,000	41,008,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				12,642,000	24,191,000	41,008,000
Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets		22,635,000		22,635,000	27,135,000
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets		19,382,000		19,382,000	21,403,000
Deferred Tax Assets, Regulatory Assets and Liabilities		54,259,000		54,259,000	57,469,000
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				0	(75,000)	0
Regulatory Settlement				(1,129,000)	0	0
Deferred Tax Liabilities, Property, Plant and Equipment		281,044,000		281,044,000	303,186,000
Deferred Tax Liabilities, Leasing Arrangements		40,133,000		40,133,000	0
Deferred Tax Liabilities Non Regulated Liabilities		410,000		410,000	537,000
Deferred Tax Liabilities, Net		309,297,000		309,297,000	294,739,000
Deferred Tax Assets, Tax Credit Carryforwards, Other		48,000		48,000	52,000
Deferred Tax Assets, Net		54,307,000		54,307,000	57,521,000
Increase (Decrease) in Prepaid Taxes		500,000	$ 7,400,000
Increase (Decrease) in Deferred Income Taxes		300,000	4,100,000
Increase (Decrease) in Regulatory Liabilities		200,000	$ 3,300,000	853,000	7,929,000	0
Effect of TCJA				0	0	(2,956,000)
Regulatory Liability, Noncurrent		625,717,000		625,717,000	611,560,000
Deferred Tax Assets, Goodwill and Intangible Assets		0		0	0
Deferred Tax Liabilities, Gross		363,604,000		363,604,000	352,260,000
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				17,438,000	19,434,000	39,624,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,716,000	4,982,000	5,072,000
Income Tax Reconciliation, Differences Regulatory Commission				(5,772,000)	1,302,000	2,357,000
Effect of TCJA				0	0	(2,956,000)
Income Tax Reconciliation, Other Adjustments				(1,188,000)	(1,184,000)	(2,619,000)
Income Tax Expense (Benefit)				$ 14,065,000	$ 24,459,000	$ 41,478,000
Effective Income Tax Rate				16.90%	26.40%	36.60%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 6,755,000	$ 9,127,000	$ 19,304,000
Current State and Local Tax Expense (Benefit)				2,101,000	3,846,000	5,956,000
Current Income Tax Expense (Benefit)				8,856,000	12,973,000	25,260,000
Deferred Federal Income Tax Expense (Benefit)				1,340,000	9,025,000	14,371,000
Deferred State and Local Income Tax Expense (Benefit)				3,869,000	2,461,000	1,847,000
Deferred Income Tax Expense (Benefit)				5,209,000	11,486,000	16,218,000
Income Tax Expense (Benefit)				14,065,000	24,459,000	41,478,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				14,065,000	24,459,000	$ 41,478,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets		2,500,000		2,500,000	2,300,000
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets		22,635,000		22,635,000	27,135,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets		19,382,000		19,382,000	21,402,000
Deferred Tax Assets, Regulatory Assets and Liabilities		54,259,000		54,259,000	57,469,000
Regulatory Liabilities		205,000,000.0		205,000,000.0	217,100,000
Regulatory Liability, Noncurrent		198,219,000		198,219,000	225,408,000
Northwest Natural Gas Company [Member] | Tax reform deferral [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Noncurrent		8,200,000		8,200,000
Excluding AFUDC [Member] | Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets		16,900,000		16,900,000	$ 19,100,000
Domestic Tax Authority [Member]
Regulatory Assets [Line Items]
Operating Loss Carryforwards		400,000		400,000
State and Local Jurisdiction [Member]
Regulatory Assets [Line Items]
Operating Loss Carryforwards		$ 400,000		$ 400,000