Significant Accounting Policies - Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 46,688	$ 41,930	$ 41,092
Regulatory Assets, Noncurrent	318,340	371,786	339,177
Northwest Natural Gas Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	46,688	41,930	41,092
Regulatory Assets, Noncurrent	318,340	371,786	339,177
Northwest Natural Gas Company [Member] | Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	4,385	12,381	11,744
Regulatory Assets, Noncurrent	2,062	3,025	3,913
Northwest Natural Gas Company [Member] | Gas costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	19,195	2,873	273
Regulatory Assets, Noncurrent	2,994	9,978	28
Northwest Natural Gas Company [Member] | Environmental costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,089	5,601	5,594
Regulatory Assets, Noncurrent	68,240	76,149	65,156
Northwest Natural Gas Company [Member] | Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	1,680	9,140	10,232
Regulatory Assets, Noncurrent	37	2,545	1,636
Northwest Natural Gas Company [Member] | Pension balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	5,009	0	0
Regulatory Assets, Noncurrent	50,080	74,173	67,527
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	168,137	174,993	171,186
Northwest Natural Gas Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9,121	9,717	11,032
Regulatory Assets, Noncurrent	$ 9,032	$ 11,738	$ 10,464