Income Tax (Details) - USD ($)		3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				$ (76,000)	$ 0		$ 0
Deferred Tax Liabilities, Property, Plant and Equipment	$ 278,735,000	$ 288,385,000		288,385,000	278,735,000
Deferred Tax Liabilities Non Regulated Liabilities	2,766,000	1,061,000		1,061,000	2,766,000
Deferred Tax Liabilities, Gross	327,062,000	337,984,000		337,984,000	327,062,000
Deferred Tax Liabilities, Net	270,526,000	280,463,000		280,463,000	270,526,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	52,000		52,000	66,000
Deferred Tax Assets, Net	$ 56,536,000	57,521,000		57,521,000	56,536,000
Federal Statutory Income Tax Rate, Percent	21.00%					35.00%
Increase (Decrease) in Regulatory Liabilities				7,929,000	0		0
Effect of TCJA				0	(3,376,000)		0
Regulatory Liability, Noncurrent	$ 586,093,000	611,560,000		611,560,000	586,093,000
Unrecognized Tax Benefits	0	0		0	0		0
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				19,222,000	39,578,000		36,901,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,927,000	5,066,000		4,844,000
Income Tax Reconciliation, Differences Regulatory Commission				1,302,000	2,357,000		2,357,000
Effect of TCJA				0	(3,376,000)		0
Income Tax Reconciliation, Other Adjustments				(1,184,000)	(2,617,000)		(1,091,000)
Income Tax Expense (Benefit)				$ 24,191,000	$ 41,008,000		$ 43,011,000
Effective Income Tax Rate				26.40%	36.30%		40.80%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 8,953,000	$ 19,345,000		$ 10,042,000
Current State and Local Tax Expense (Benefit)				3,785,000	5,963,000		3,116,000
Current Income Tax Expense (Benefit)				12,738,000	25,308,000		13,158,000
Deferred Federal Income Tax Expense (Benefit)				9,001,000	13,869,000		25,473,000
Deferred State and Local Income Tax Expense (Benefit)				2,452,000	1,831,000		4,380,000
Deferred Income Tax Expense (Benefit)				11,453,000	15,700,000		29,853,000
Income Tax Expense (Benefit)				24,191,000	41,008,000		43,011,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				24,191,000	41,008,000		43,011,000
Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	23,352,000	27,135,000		27,135,000	23,352,000
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	22,209,000	21,403,000		21,403,000	22,209,000
Deferred Tax Assets, Regulatory Assets and Liabilities	56,470,000	57,469,000		57,469,000	56,470,000
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Deferred Tax Rate Differential, Post TCJA				(75,000)	0		0
Deferred Tax Liabilities, Property, Plant and Equipment	296,113,000	303,186,000		303,186,000	296,113,000
Deferred Tax Liabilities Non Regulated Liabilities	2,250,000	537,000		537,000	2,250,000
Deferred Tax Liabilities, Gross	343,924,000	352,260,000		352,260,000	343,924,000
Deferred Tax Liabilities, Net	287,388,000	294,739,000		294,739,000	287,388,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	66,000	52,000		52,000	66,000
Deferred Tax Assets, Net	56,536,000	57,521,000		57,521,000	56,536,000
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	7,929,000	0		0
Effect of TCJA				0	(2,956,000)		0
Regulatory Liability, Noncurrent	586,093,000	611,560,000		611,560,000	586,093,000
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate				19,434,000	39,624,000		37,137,000
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit				4,982,000	5,072,000		4,858,000
Income Tax Reconciliation, Differences Regulatory Commission				1,302,000	2,357,000		2,357,000
Effect of TCJA				0	(2,956,000)		0
Income Tax Reconciliation, Other Adjustments				(1,184,000)	(2,619,000)		(1,077,000)
Income Tax Expense (Benefit)				$ 24,459,000	$ 41,478,000		$ 43,275,000
Effective Income Tax Rate				26.40%	36.60%		40.80%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)				$ 9,127,000	$ 19,304,000		$ 10,158,000
Current State and Local Tax Expense (Benefit)				3,846,000	5,956,000		3,131,000
Current Income Tax Expense (Benefit)				12,973,000	25,260,000		13,289,000
Deferred Federal Income Tax Expense (Benefit)				9,025,000	14,371,000		25,581,000
Deferred State and Local Income Tax Expense (Benefit)				2,461,000	1,847,000		4,405,000
Deferred Income Tax Expense (Benefit)				11,486,000	16,218,000		29,986,000
Income Tax Expense (Benefit)				24,459,000	41,478,000		43,275,000
Tax Provision Regulated And Nonutility [Abstract]
Income Tax Expense (Benefit)				24,459,000	41,478,000		$ 43,275,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets	900,000	2,300,000		2,300,000	900,000
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	23,352,000	27,135,000		27,135,000	23,352,000
Northwest Natural Gas Company [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Deferred Tax Liabilities, Regulatory Assets	22,209,000	21,402,000		21,402,000	22,209,000
Deferred Tax Assets, Regulatory Assets and Liabilities	56,470,000	57,469,000		57,469,000	56,470,000
Regulatory Liabilities	213,300,000	217,100,000		217,100,000	213,300,000
Regulatory Liability, Noncurrent	213,306,000	225,408,000		225,408,000	213,306,000
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	$ 21,300,000	$ 19,100,000		$ 19,100,000	$ 21,300,000