Significant Accounting Policies - Regulatory Asset Disclosure - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 47,789	$ 31,745	$ 37,815
Regulatory Assets, Noncurrent	338,692	348,927	328,024
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	40	40	0
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	47,789	31,745	37,815
Regulatory Assets, Noncurrent	338,652	348,887	328,024
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	1,038	4,198	4,845
Regulatory Assets, Noncurrent	1,272	2,852	939
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	21,851	1,979	10,898
Regulatory Assets, Noncurrent	11,242	3,925	350
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,396	4,992	4,459
Regulatory Assets, Noncurrent	84,977	90,623	82,491
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4	361	610
Regulatory Assets, Noncurrent	2	1,031	0
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	7,131	7,131	6,794
Regulatory Assets, Noncurrent	41,111	43,383	46,031
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,273	3,484	3,576
Regulatory Assets, Noncurrent	13,895	15,368	16,354
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	165,598	170,812	168,676
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	10,096	9,600	6,633
Regulatory Assets, Noncurrent	$ 20,555	$ 20,893	$ 13,183