Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 47,137	$ 44,657	$ 46,770
Regulatory Liability, Noncurrent	623,219	625,717	600,698
COVID-19 Incremental Costs	700
COVID-19 Incremental Costs, Bad Debt	400
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	844	0	0
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	47,137	44,657	46,770
Regulatory Liability, Noncurrent	622,375	625,717	600,698
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,046	1,223	11,126
Regulatory Liability, Noncurrent	1,328	2,013	1,421
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,257	6,622	7,284
Regulatory Liability, Noncurrent	2,451	3,337	541
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	11,203	4,831	2,055
Regulatory Liability, Noncurrent	4,784	6,378	614
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,522	8,435	7,763
Regulatory Liability, Noncurrent	192,644	198,219	202,692
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	22,109	23,546	18,542
Regulatory Liability, Noncurrent	12,956	13,877	10,728
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 408,212	$ 401,893	$ 384,702