Significant Accounting Policies - Regulatory Liability Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 32,484	$ 47,436	$ 34,275
Regulatory Liability, Noncurrent	605,036	611,560	602,294
Northwest Natural Gas Company [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	32,484	47,436	34,275
Regulatory Liability, Noncurrent	605,036	611,560	602,294
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	5,630	17,182	20,906
Regulatory Liability, Noncurrent	226	552	3,460
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,944	8,740	1,938
Regulatory Liability, Noncurrent	670	725	1,077
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	857	2,264	2,153
Regulatory Liability, Noncurrent	74	0	410
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,763	0	0
Regulatory Liability, Noncurrent	202,422	225,408	222,734
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	16,290	19,250	9,278
Regulatory Liability, Noncurrent	11,299	4,411	4,368
Northwest Natural Gas Company [Member] | Accrued asset removal costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 390,345	$ 380,464	$ 370,245