Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 37,815	$ 41,929	$ 46,317
Regulatory Assets, Noncurrent	328,024	343,146	327,194
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	37,815	41,929	46,317
Regulatory Assets, Noncurrent	328,024	343,146	327,194
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,845	2,000	2,845
Regulatory Assets, Noncurrent	939	609	1,161
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	10,898	20,140	17,927
Regulatory Assets, Noncurrent	350	2,866	8,919
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,459	4,762	5,090
Regulatory Assets, Noncurrent	82,491	87,624	66,612
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	610	1,969	3,937
Regulatory Assets, Noncurrent	0	0	250
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	6,794	5,939	4,955
Regulatory Assets, Noncurrent	46,031	48,251	50,408
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	3,576	2,209	2,209
Regulatory Assets, Noncurrent	16,354	17,173	17,758
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	168,676	173,262	171,565
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	6,633	4,910	9,354
Regulatory Assets, Noncurrent	$ 13,183	$ 13,361	$ 10,521