Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 46,770	$ 47,436	$ 34,946
Regulatory Liability, Noncurrent	600,698	611,560	600,442
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	46,770	47,436	34,946
Regulatory Liability, Noncurrent	600,698	611,560	600,442
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	11,126	17,182	17,798
Regulatory Liability, Noncurrent	1,421	552	5,639
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,284	8,740	1,120
Regulatory Liability, Noncurrent	541	725	1,148
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,055	2,264	2,501
Regulatory Liability, Noncurrent	614	0	1,253
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,763	0	0
Regulatory Liability, Noncurrent	202,692	225,408	219,795
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	18,542	19,250	13,527
Regulatory Liability, Noncurrent	10,728	4,411	7,244
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 384,700	$ 380,500	$ 365,400