Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 41,126	$ 44,657	$ 32,484
Regulatory Liability, Noncurrent	632,400	625,717	605,036
COVID-19 Incremental Costs	4,000
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	869	0	0
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	41,126	44,657	32,484
Regulatory Liability, Noncurrent	631,531	625,717	605,036
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	3,767	1,223	5,630
Regulatory Liability, Noncurrent	538	2,013	226
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	5,950	6,622	1,944
Regulatory Liability, Noncurrent	3,958	3,337	670
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	11,498	4,831	857
Regulatory Liability, Noncurrent	2,108	6,378	74
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,098	8,435	7,763
Regulatory Liability, Noncurrent	193,414	198,219	202,422
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	12,813	23,546	16,290
Regulatory Liability, Noncurrent	16,794	13,877	11,299
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 414,719	$ 401,893	$ 390,345