Significant Accounting Policies - Regulatory Asset Disclosure - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 29,740	$ 41,929	$ 47,996
Regulatory Assets, Noncurrent	324,176	343,146	313,890
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	40	0	0
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	29,740	41,929	47,996
Regulatory Assets, Noncurrent	324,136	343,146	313,890
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	1,459	2,000	4,156
Regulatory Assets, Noncurrent	921	609	2,998
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,207	20,140	21,094
Regulatory Assets, Noncurrent	110	2,866	921
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,440	4,762	5,100
Regulatory Assets, Noncurrent	84,567	87,624	69,250
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	155	1,969	413
Regulatory Assets, Noncurrent	6	0	0
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	7,131	5,939	5,215
Regulatory Assets, Noncurrent	45,203	48,251	50,019
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,208	2,209	2,209
Regulatory Assets, Noncurrent	16,792	17,173	17,758
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	159,207	173,262	164,709
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,140	4,910	9,809
Regulatory Assets, Noncurrent	$ 17,330	$ 13,361	$ 8,235