Significant Accounting Policies - Regulatory Asset Disclosure - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 60,672	$ 31,745	$ 30,021
Regulatory Assets, Noncurrent	330,710	348,927	324,358
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	40	40	38
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	60,672	31,745	30,021
Regulatory Assets, Noncurrent	330,670	348,887	324,320
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	3,393	4,198	2,956
Regulatory Assets, Noncurrent	453	2,852	1,658
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	29,486	1,979	6,046
Regulatory Assets, Noncurrent	5,742	3,925	94
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,688	4,992	4,176
Regulatory Assets, Noncurrent	85,423	90,623	81,757
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	1,011	361	85
Regulatory Assets, Noncurrent	198	1,031	0
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	7,131	7,131	7,131
Regulatory Assets, Noncurrent	40,342	43,383	45,315
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,345	3,484	2,208
Regulatory Assets, Noncurrent	13,903	15,368	17,608
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	160,385	170,812	164,091
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	11,618	9,600	7,419
Regulatory Assets, Noncurrent	$ 24,224	$ 20,893	$ 13,797