Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 41,930	$ 45,781
Regulatory Assets, Noncurrent	371,786	356,608
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	41,930	45,781
Regulatory Assets, Noncurrent	371,786	356,608
Northwest Natural Gas Company [Member] | Unrealized loss on derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	12,381	18,712
Regulatory Assets, Noncurrent	3,025	4,649
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,873	154
Regulatory Assets, Noncurrent	9,978	84
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,601	6,198
Regulatory Assets, Noncurrent	76,149	72,128
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,140	11,227
Regulatory Assets, Noncurrent	2,545	3,970
Northwest Natural Gas Company [Member] | Income taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	2,218	2,218
Regulatory Assets, Noncurrent	19,185	19,991
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	74,173	60,383
Northwest Natural Gas Company [Member] | Pension and other postretirement benefit liabilities [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	174,993	179,824
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,717	7,272
Regulatory Assets, Noncurrent	$ 11,738	$ 15,579