Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 302,493	$ 263,931
Revenues	285,348	263,635
Operating leases	48
Natural Gas Sales [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	296,186	258,229
Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,783	2,577
Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,506	1,579
Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,516	1,546
Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	502	0
Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	(17,253)	(372)
Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating leases	108	76
Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenues	284,846	263,635
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	301,991	263,931
Revenues	284,846	263,635
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Natural Gas Sales [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	296,186	258,229
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,783	2,577
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,506	1,579
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,516	1,546
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	(17,253)	(372)
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating leases	108	76
Operating Segments [Member] | Natural Gas Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenues	279,041	257,933
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	296,186	258,229
Revenues	279,041	257,933
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Natural Gas Sales [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	296,186	258,229
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	(17,253)	(372)
Operating Segments [Member] | Natural Gas Distribution [Member] | Northwest Natural Gas Company [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating leases	108	76
Operating Segments [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenues	284,846	263,635
Corporate, Non-Segment [Member] | Other Northwest Natural [Member]
Disaggregation of Revenue [Line Items]
Revenues	5,805	5,702
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5,805	5,702
Revenues	5,805	5,702
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Natural Gas Sales [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,783	2,577
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,506	1,579
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,516	1,546
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	0	0
Corporate, Non-Segment [Member] | Other Northwest Natural [Member] | Northwest Natural Gas Company [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating leases	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	502	0
Revenues	502	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Natural Gas Sales [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Gas Storage Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Appliance Center Revenue [Member] | Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Other revenue [Member] | Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	502	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Alternative revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	0	0
Corporate, Non-Segment [Member] | Other Northwest Holdings [Member] | Leasing revenue [Member]
Disaggregation of Revenue [Line Items]
Operating leases	$ 0	$ 0