Significant Accounting Policies - Regulatory Liability Disclosure - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 59,236	$ 44,657	$ 37,573
Regulatory Liability, Noncurrent	649,521	625,717	615,813
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	870	0	0
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	59,236	44,657	37,573
Regulatory Liability, Noncurrent	648,651	625,717	615,813
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,625	1,223	3,204
Regulatory Liability, Noncurrent	69	2,013	50
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	23,738	6,622	5,720
Regulatory Liability, Noncurrent	12,921	3,337	1,610
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	12,702	4,831	2,007
Regulatory Liability, Noncurrent	874	6,378	80
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	6,900	8,435	7,763
Regulatory Liability, Noncurrent	196,558	198,219	206,572
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	14,271	23,546	18,879
Regulatory Liability, Noncurrent	16,876	13,877	11,083
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 421,353	$ 401,893	$ 396,418