Pension and Other Postretirement Benefits Costs - Schedule of AOCL and Changes in AOCL, Non-Qualified Employee Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (8,438)
Ending balance	(7,188)	$ (8,438)
Northwest Holdings and Northwest Natural [Member] | Non-Qualified Pension Plans, Defined Benefit [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8,438)	(6,951)
Amounts reclassified to AOCL	642	(2,794)
Amortization of actuarial losses	1,052	946
Total reclassifications before tax	1,694	(1,848)
Tax expense (benefit)	(444)	361
Total reclassifications for the period	1,250	(1,487)
Ending balance	$ (7,188)	$ (8,438)