Segment Information - Schedule of Summarized Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 90,317	$ 91,239	$ 499,108	$ 479,441
Depreciation and amortization	23,375	21,485	67,334	63,507
Income from operations	(9,634)	(6,111)	82,509	69,909
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(18,506)	(11,144)	26,963	30,528
Capital expenditures			152,993	158,795
Total assets	3,251,618	3,081,407	3,251,618	3,081,407	$ 3,242,662
Discontinued operations - current assets	14,612	12,644	14,612	12,644	13,269
Discontinued operations - total assets		12,600		12,600
Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	83,063	85,077	480,088	461,525
Depreciation and amortization	22,830	21,127	66,071	62,436
Income from operations	(11,580)	(9,780)	75,975	59,521
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(19,570)	(11,983)	22,848	24,930
Capital expenditures	60,596	55,914	151,114	156,609
Total assets	3,101,372	2,972,395	3,101,372	2,972,395	3,141,969
Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,529	6,150	14,592	17,900
Depreciation and amortization	244	327	750	976
Income from operations	2,761	3,499	9,090	10,341
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	1,982	708	6,513	5,538
Capital expenditures	644	511	801	2,186
Total assets	48,269	51,886	48,269	51,886	50,767
Northwest Natural Gas Company [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	87,592	91,227	494,680	479,425
Depreciation and amortization	23,074	21,454	66,821	63,412
Income from operations	(8,819)	(6,281)	85,065	69,862
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(17,588)	(11,275)	29,361	30,468
Capital expenditures	61,240	56,425	151,915	158,795
Total assets	3,149,641	3,024,281	3,149,641	3,024,281	3,192,736
Other Northwest Holdings [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,725	12	4,428	16
Depreciation and amortization	301	31	513	95
Income from operations	(815)	170	(2,556)	47
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(918)	131	(2,398)	60
Capital expenditures	606	0	1,078	0
Total assets	87,365	44,482	87,365	44,482	36,657
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	87,592	91,227	494,680	479,425
Depreciation and amortization	23,074	21,454	66,821	63,412
Income from operations	(8,819)	(6,281)	85,065	69,862
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(17,588)	(11,275)	29,361	30,468
Capital expenditures			151,915	158,795
Total assets	3,149,641	3,085,587	3,149,641	3,085,587	3,192,736
Discontinued operations - current assets	0	26,227	0	26,227	0
Discontinued operations - total assets	0	1,930	0	1,930	0
Northwest Natural Gas Company [Member] | Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	83,063	85,077	480,088	461,525
Northwest Natural Gas Company [Member] | Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,529	6,150	14,592	17,900
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	87,592	91,227	494,680	479,425
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Discontinued operations - total assets		61,300		61,300
Parent Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	90,317	91,239	499,108	479,441
Depreciation and amortization	23,375	21,485	67,334	63,507
Income from operations	(9,634)	(6,111)	82,509	69,909
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(18,506)	(11,144)	26,963	30,528
Capital expenditures	61,846	56,425	152,993	158,795
Total assets	$ 3,237,006	$ 3,068,763	$ 3,237,006	$ 3,068,763	$ 3,229,393