Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 47,996	$ 41,930	$ 41,241
Regulatory Assets, Noncurrent	313,890	371,786	333,917
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	47,996	41,930	41,241
Regulatory Assets, Noncurrent	313,890	371,786	333,917
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,156	12,381	8,828
Regulatory Assets, Noncurrent	2,998	3,025	3,016
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	21,094	2,873	461
Regulatory Assets, Noncurrent	921	9,978	14
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,100	5,601	5,633
Regulatory Assets, Noncurrent	69,250	76,149	63,464
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	413	9,140	11,990
Regulatory Assets, Noncurrent	0	2,545	829
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,215	0	0
Regulatory Assets, Noncurrent	50,019	74,173	72,291
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,209	2,218	2,217
Regulatory Assets, Noncurrent	17,758	19,185	19,267
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	164,709	174,993	165,741
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,809	9,717	12,112
Regulatory Assets, Noncurrent	$ 8,235	$ 11,738	$ 9,295