Significant Accounting Policies - Leases ASU 2016-02 (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	$ 5,013,000	$ 0	$ 0
Operating Lease, Liability	4,992,000
Property, plant, and equipment	3,355,811,000	3,414,490,000	3,298,856,000
Other Liabilities, Noncurrent	129,835,000	147,763,000	140,163,000
Northwest Natural Gas Company [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	4,797,000	0	0
Operating Lease, Liability	4,775,000
Property, plant, and equipment	3,337,005,000	3,410,439,000	3,298,481,000
Other Liabilities, Noncurrent	129,736,000	147,668,000	$ 140,073,000
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