Significant Accounting Policies - Leases ASU 2016-02 (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	$ 3,917,000	$ 0	$ 0
Operating Lease, Liability	3,903,000
Property, plant, and equipment	3,416,718,000	3,414,490,000	3,370,388,000
Other Liabilities, Noncurrent	123,352,000	147,763,000	140,475,000
Northwest Natural Gas Company [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	3,745,000	0	0
Operating Lease, Liability	3,729,000
Property, plant, and equipment	3,397,053,000	3,410,439,000	3,366,896,000
Other Liabilities, Noncurrent	123,259,000	147,668,000	$ 140,390,000
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Initial Direct or Capitalized Lease Cost to be De-recognized, Legacy Leases	0
Operating lease right of use asset	7,300,000
Operating Lease, Liability	7,300,000
Accounting Standards Update 2016-02 [Member] | Northwest Natural Gas Company [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	7,000,000.0
Operating Lease, Liability	$ 7,000,000
Accounting Standards Update 2016-02 [Member] | New Headquarters Lease [Member] | Build to Suit Lease [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant, and equipment		26,000,000.0
Other Liabilities, Noncurrent		$ 26,000,000