Segment Information - Schedule of Summarized Financial Information by Segment - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 93,284	$ 90,317	$ 513,406	$ 499,108
Depreciation and amortization	25,934	23,375	76,445	67,334
Income from operations	(11,588)	(9,634)	73,800	82,509
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(18,677)	(18,506)	24,467	26,963
Total assets	3,618,097	3,251,618	3,618,097	3,251,618	$ 3,428,454
Discontinued operations - total assets	16,900	14,600	16,900	14,600	15,100
Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	83,761	83,063	489,240	480,088
Depreciation and amortization	25,433	22,830	74,117	66,071
Income from operations	(16,196)	(11,580)	65,496	75,975
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(22,120)	(19,570)	19,476	22,848
Capital expenditures	69,213	60,596	186,842	151,114
Total assets	3,451,771	3,101,372	3,451,771	3,101,372	3,273,835
Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,606	4,529	12,813	14,592
Depreciation and amortization	248	244	740	750
Income from operations	2,727	2,761	6,955	9,090
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	1,900	1,982	4,771	6,513
Capital expenditures	1,113	644	1,711	801
Total assets	11,596	48,269	11,596	48,269	47,652
Northwest Natural Gas Company [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	88,367	87,592	502,053	494,680
Depreciation and amortization	25,681	23,074	74,857	66,821
Income from operations	(13,469)	(8,819)	72,451	85,065
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(20,220)	(17,588)	24,247	29,361
Capital expenditures	70,326	61,240	188,553	151,915
Total assets	3,463,367	3,149,641	3,463,367	3,149,641	3,321,487
Other Northwest Holdings [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,917	2,725	11,353	4,428
Depreciation and amortization	253	301	1,588	513
Income from operations	1,881	(815)	1,349	(2,556)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	1,543	(918)	220	(2,398)
Capital expenditures	728	606	4,783	1,078
Total assets	137,802	87,365	137,802	87,365	91,833
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	88,367	87,592	502,053	494,680
Depreciation and amortization	25,681	23,074	74,857	66,821
Income from operations	(13,469)	(8,819)	72,451	85,065
Total assets	3,463,367	3,149,641	3,463,367	3,149,641	3,321,487
Northwest Natural Gas Company [Member] | Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	83,761	83,063	489,240	480,088
Northwest Natural Gas Company [Member] | Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,606	4,529	12,813	14,592
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	88,367	87,592	502,053	494,680
Parent Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	93,284	90,317	513,406	499,108
Depreciation and amortization	25,934	23,375	76,445	67,334
Income from operations	(11,588)	(9,634)	73,800	82,509
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(18,677)	(18,506)	24,467	26,963
Capital expenditures	71,054	61,846	193,336	152,993
Total assets	$ 3,601,169	$ 3,237,006	$ 3,601,169	$ 3,237,006	$ 3,413,320