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	28,745		28,745		28,745		28,745
Dividends on common stock			(41,035)				(41,035)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	29,206			461	29,206			461
Stock-based compensation		2,161				2,161
Shares issued pursuant to equity based plans		3,893				3,893
Capital contribution from parent					0	0
Stock Issued During Period, Value, New Issues		0
Ending Balance at Sep. 30, 2018	$ 737,581	455,499	290,059	(7,977)	737,581	455,499	290,059	(7,977)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased During Period, Value		(7,945)				(7,945)
Dividends per share of common stock	$ 1.4175
Business Combination, Consideration Transferred		8,525				8,525
Beginning 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]
Net income	(11,794)		(11,794)		(11,794)		(11,794)
Dividends on common stock			(13,609)				(13,609)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	(11,640)			154	(11,640)			154
Stock-based compensation		237				237
Shares issued pursuant to equity based plans		2,487				2,487
Capital contribution from parent						0
Stock Issued During Period, Value, New Issues		0
Ending Balance at Sep. 30, 2018	$ 737,581	455,499	290,059	(7,977)	737,581	455,499	290,059	(7,977)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased During Period, Value		(7,945)				(7,945)
Dividends per share of common stock	$ 0.4725
Business Combination, Consideration Transferred		8,525				8,525
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	24,995		24,995		29,361		29,361
Dividends on common stock			(42,287)				(39,601)
Stranded Deferred Tax Remeasurement Benefit	1,366
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			1,366	(1,366)			1,366	(1,366)
Other comprehensive income	25,339			344	29,705			344
Stock-based compensation		2,309				0
Shares issued pursuant to equity based plans		3,718				0
Capital contribution from parent					93,182	93,182
Stock Issued During Period, Value, New Issues	93,000	92,956
Ending Balance at Sep. 30, 2019	$ 844,669	556,623	296,256	(8,210)	798,954	319,634	487,530	(8,210)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased During Period, Value		0				0
Dividends per share of common stock	$ 1.4250
Business Combination, Consideration Transferred		0				0
Beginning 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]
Net income	(19,301)		(19,301)		(17,588)		(17,588)
Dividends on common stock			(14,461)				(13,687)
Stranded Deferred Tax Remeasurement Benefit	0
Stranded Deferred Tax Remeasurement Benefit | Accounting Standards Update 2018-02 [Member]			0	0			0	0
Other comprehensive income	(19,187)			114	(17,474)			114
Stock-based compensation		485				0
Shares issued pursuant to equity based plans		1,082				0
Capital contribution from parent						0
Stock Issued During Period, Value, New Issues		4
Ending Balance at Sep. 30, 2019	$ 844,669	556,623	$ 296,256	$ (8,210)	$ 798,954	319,634	$ 487,530	$ (8,210)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased During Period, Value		0				0
Dividends per share of common stock	$ 0.4750
Business Combination, Consideration Transferred		$ 0				$ 0