Significant Accounting Policies - Leases ASU 2016-02 (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	$ 6,163,000	$ 0	$ 0
Operating Lease, Liability	6,151,000
Property, plant, and equipment	3,439,460,000	3,414,490,000	3,251,075,000
Other Liabilities, Noncurrent	120,569,000	147,763,000	137,147,000
Northwest Natural Gas Company [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right of use asset	5,903,000	0	0
Operating Lease, Liability	5,890,000
Property, plant, and equipment	3,435,304,000	3,410,439,000	3,250,700,000
Other Liabilities, Noncurrent	120,465,000	147,668,000	$ 137,059,000
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
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
Other Liabilities, Noncurrent		$ 26,000,000