Pension and Other Postretirement Benefits Costs - Changes in Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	$ 287,925
Fair value of plan assets at December 31	257,797	$ 287,925
Northwest Natural Gas Company [Member] | Other Postretirement Benefits Plan [Member]
Reconciliation of change in benefit obligation:
Obligation at January 1	28,927	29,395
Service cost	282	341
Interest cost	964	1,141
Net actuarial (gain) loss	327	213
Benefits paid	(1,674)	(1,737)	$ (1,732)
Obligation at December 31	28,172	28,927	29,395
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Employer contributions	1,674	1,737
Benefits paid	(1,674)	(1,737)	(1,732)
Fair value of plan assets at December 31	0	0	0
Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Obligation at January 1	486,289	457,839
Service cost	7,185	7,090
Interest cost	16,991	18,111
Net actuarial (gain) loss	32,979	(34,829)
Benefits paid	(21,918)	(31,580)	(20,959)
Obligation at December 31	455,568	486,289	457,839
Reconciliation of change in plan assets:
Fair value of plan assets at January 1	287,925	257,714
Actual return on plan assets	(25,925)	40,308
Employer contributions	17,715	21,483
Benefits paid	(21,918)	(31,580)	(20,959)
Fair value of plan assets at December 31	257,797	287,925	$ 257,714
Northwest Holdings and Northwest Natural [Member] | Other Postretirement Benefits Plan [Member]
Reconciliation of change in plan assets:
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(28,172)	(28,927)
Northwest Holdings and Northwest Natural [Member] | Pension Plan [Member]
Reconciliation of change in plan assets:
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (197,771)	(198,364)
Partial Buy-out, Assets Transferred to Insurer [Member] | Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Benefits paid		(9,300)
Reconciliation of change in plan assets:
Benefits paid		(9,300)
Partial Buy-out, Liabilities Transferred to Insurer [Member] | Northwest Natural Gas Company [Member] | Pension Plan [Member]
Reconciliation of change in benefit obligation:
Benefits paid		(8,700)
Reconciliation of change in plan assets:
Benefits paid		$ (8,700)