Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock Including APIC [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Northwest Natural Gas Company [Member]	Northwest Natural Gas Company [Member] Common Stock Including APIC [Member]	Northwest Natural Gas Company [Member] Retained Earnings [Member]	Northwest Natural Gas Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2017	$ 742,776	$ 448,865	$ 302,349	$ (8,438)	$ 742,776	$ 448,865	$ 302,349	$ (8,438)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,539		40,539		40,539		40,539
Dividends on common stock			(27,426)				(27,426)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	40,846			307	40,846			307
Stock-based compensation		1,924				1,924
Shares issued pursuant to equity based plans		1,406				1,406
Capital contribution from parent					0	0
Stock Issued During Period, Value, New Issues		0
Ending Balance at Jun. 30, 2018	$ 759,526	452,195	315,462	(8,131)	759,526	452,195	315,462	(8,131)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per share of common stock	$ 0.9450
Beginning Balance at Mar. 31, 2018	$ 772,205	450,408	330,081	(8,284)	772,205	450,408	330,081	(8,284)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(998)		(998)		(998)		(998)
Dividends on common stock			(13,621)				(13,621)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	(845)			153	(845)			153
Stock-based compensation		752				752
Shares issued pursuant to equity based plans		1,035				1,035
Capital contribution from parent						0
Stock Issued During Period, Value, New Issues		0
Ending Balance at Jun. 30, 2018	$ 759,526	452,195	315,462	(8,131)	759,526	452,195	315,462	(8,131)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per share of common stock	$ 0.4725
Beginning Balance at Dec. 31, 2018	$ 762,634	457,640	312,182	(7,188)	715,668	226,452	496,404	(7,188)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	44,296		44,296		46,949		46,949
Dividends on common stock			(27,826)				(25,914)
Stranded Deferred Tax Remeasurement Benefit	1,366
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]	1,400		1,366	(1,366)			1,366	(1,366)
Other comprehensive income	44,526			230	47,179			230
Stock-based compensation		1,824				0
Shares issued pursuant to equity based plans		2,636				0
Capital contribution from parent					93,182	93,182
Stock Issued During Period, Value, New Issues	92,800	92,952
Ending Balance at Jun. 30, 2019	$ 876,746	555,052	330,018	(8,324)	830,115	319,634	518,805	(8,324)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per share of common stock	$ 0.9500
Beginning Balance at Mar. 31, 2019	$ 794,227	459,932	342,734	(8,439)	746,743	226,452	528,730	(8,439)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,095		1,095		3,054		3,054
Dividends on common stock			(13,811)				(12,979)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	1,210			115	3,169			115
Stock-based compensation		579				0
Shares issued pursuant to equity based plans		1,589				0
Capital contribution from parent						93,182
Stock Issued During Period, Value, New Issues		92,952
Ending Balance at Jun. 30, 2019	$ 876,746	$ 555,052	$ 330,018	$ (8,324)	$ 830,115	$ 319,634	$ 518,805	$ (8,324)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends per share of common stock	$ 0.4750