Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant, and equipment	$ 3,439,460	$ 3,414,490	$ 3,251,075
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	1,005,117	993,118	972,773
Property, Plant and Equipment, Net	2,434,343	2,421,372	2,278,302
Other Liabilities, Noncurrent	120,569	147,763	137,147
Natural Gas Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment	3,159,754	3,134,122	3,003,962
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	204,938	204,978	177,133
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	985,961	974,252	954,858
Property, Plant and Equipment, Net	2,378,731	2,364,848	2,226,237
Other Northwest Natural [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment	65,283	66,009	65,353
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	5,329	5,330	4,252
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	18,851	18,603	17,715
Property, Plant and Equipment, Net	51,761	52,736	51,890
Northwest Natural Gas Company [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	2,430,492	2,417,584	2,278,127
Other Northwest Holdings [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment	4,156	4,051	375
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	305	263	200
Property, Plant and Equipment, Net	3,851	3,788	175
Northwest Holdings and Northwest Natural [Member]
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment non-cash	$ 25,035	23,676	$ 21,855
New Headquarters Lease [Member]
Property, Plant and Equipment [Line Items]
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	20 years
New Headquarters Lease [Member] | Accounting Standards Update 2016-02 [Member] | Build to Suit Lease [Member]
Property, Plant and Equipment [Line Items]
Property, plant, and equipment		26,000
Other Liabilities, Noncurrent		$ 26,000