Pension and Other Postretirement Benefits Costs - Schedule of Components of Net Periodic Benefit Cost for NW Natural (Details) - Northwest Natural Gas Company Consolidated [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 282	$ 341	$ 391
Interest cost	964	1,141	1,175
Expected return on plan assets	0	0	0
Amortization of prior service costs	468	468	468
Amortization of net actuarial loss	448	696	705
Settlement expense	0	0	0
Net periodic benefit cost	1,226	1,710	1,803
Amount allocated to construction	(98)	(587)	(600)
Amount deferred to regulatory balancing account	0	0	0
Net amount charged to expense	1,128	1,123	1,203
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7,185	7,090	7,083
Interest cost	16,991	18,111	18,399
Expected return on plan assets	20,639	20,433	20,054
Amortization of prior service costs	(43)	(127)	(231)
Amortization of net actuarial loss	19,813	15,748	14,624
Settlement expense	0	0	(193)
Net periodic benefit cost	23,393	20,643	20,476
Amount allocated to construction	(2,764)	(6,597)	(5,746)
Amount deferred to regulatory balancing account	(10,314)	(6,542)	(6,252)
Net amount charged to expense	$ 10,315	$ 7,504	$ 8,478