Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 30,021	$ 41,929	$ 46,688
Regulatory Assets, Noncurrent	324,358	343,146	318,340
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	38	0	0
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	30,021	41,929	46,688
Regulatory Assets, Noncurrent	324,320	343,146	318,340
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,956	2,000	4,385
Regulatory Assets, Noncurrent	1,658	609	2,062
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	6,046	20,140	19,195
Regulatory Assets, Noncurrent	94	2,866	2,994
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,176	4,762	5,089
Regulatory Assets, Noncurrent	81,757	87,624	68,240
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	85	1,969	1,680
Regulatory Assets, Noncurrent	0	0	37
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	7,131	5,939	5,009
Regulatory Assets, Noncurrent	45,315	48,251	50,080
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,208	2,209	2,209
Regulatory Assets, Noncurrent	17,608	17,173	17,758
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	164,091	173,262	168,137
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	7,419	4,910	9,121
Regulatory Assets, Noncurrent	$ 13,797	$ 13,361	$ 9,032