Segment Information - Schedule of Summarized Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 134,971	$ 123,443	$ 420,122	$ 408,791
Depreciation and amortization	25,836	22,387	50,511	43,959
Income from operations	8,942	16,098	85,388	92,143
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(5,132)	2,051	43,144	45,469
Total assets	3,641,347	3,238,131	3,641,347	3,238,131	$ 3,428,454
Discontinued operations - total assets	16,400	14,000	16,400	14,000	15,100
Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	126,992	117,984	405,479	397,025
Depreciation and amortization	24,738	21,992	48,684	43,241
Income from operations	6,434	14,657	81,692	87,555
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(6,347)	1,212	41,596	42,418
Capital expenditures	61,472	41,912	117,629	90,518
Total assets	3,470,306	3,089,944	3,470,306	3,089,944	3,273,835
Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,165	4,258	8,207	10,063
Depreciation and amortization	248	251	492	506
Income from operations	2,465	2,583	4,228	6,329
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	1,635	1,842	2,871	4,531
Capital expenditures	521	105	598	157
Total assets	10,398	48,749	10,398	48,749	47,652
Northwest Natural Gas Company [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	131,157	122,242	413,686	407,088
Depreciation and amortization	24,986	22,243	49,176	43,747
Income from operations	8,899	17,240	85,920	93,884
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(4,712)	3,054	44,467	46,949
Capital expenditures	61,993	42,017	118,227	90,675
Total assets	3,480,704	3,138,693	3,480,704	3,138,693	3,321,487
Other Northwest Holdings [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,814	1,201	6,436	1,703
Depreciation and amortization	850	144	1,335	212
Income from operations	43	(1,142)	(532)	(1,741)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(420)	(1,003)	(1,323)	(1,480)
Capital expenditures	2,843	366	4,055	472
Total assets	144,251	85,437	144,251	85,437	91,833
Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	131,157	122,242	413,686	407,088
Depreciation and amortization	24,986	22,243	49,176	43,747
Income from operations	8,899	17,240	85,920	93,884
Total assets	3,480,704	3,138,693	3,480,704	3,138,693	3,321,487
Northwest Natural Gas Company [Member] | Natural Gas Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	126,992	117,984	405,479	397,025
Northwest Natural Gas Company [Member] | Other Northwest Natural [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,165	4,258	8,207	10,063
Northwest Natural Gas Company [Member] | Northwest Natural Gas Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	131,157	122,242	413,686	407,088
Parent Company [Member]
Segment Reporting Information [Line Items]
Operating revenues	134,971	123,443	420,122	408,791
Depreciation and amortization	25,836	22,387	50,511	43,959
Income from operations	8,942	16,098	85,388	92,143
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(5,132)	2,051	43,144	45,469
Capital expenditures	64,836	42,383	122,282	91,147
Total assets	$ 3,624,955	$ 3,224,130	$ 3,624,955	$ 3,224,130	$ 3,413,320