Significant Accounting Policies - Revenue from Contract with Customer (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 226,702	$ 91,239	$ 124,567	$ 263,635	$ 238,626	$ 86,212	$ 134,476	$ 295,724	$ 706,143	$ 755,038	$ 668,173
Operating Expenses									573,981	604,136	517,456
Revenue taxes									30,082	0	0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues									676,000
Operating Expenses									543,900
Northwest Holdings and Northwest Natural [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes									30,100	32,200	28,300
Northwest Holdings and Northwest Natural [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues									30,100
Operating Expenses									$ 30,100
Northwest Holdings and Northwest Natural [Member] | Revenue Taxes in Operating Revenues [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes										19,100	17,100
Northwest Holdings and Northwest Natural [Member] | Revenue Taxes on Balance Sheet [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue taxes										$ 13,100	$ 11,200