Segment Information (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Operating revenues		$ 99,183	$ 138,280	$ 354,712	$ 399,945
Cost of Gas		20,871	62,176	129,282	187,881
Environmental Remediation Expense		1,893	0	6,922	0
Depreciation and amortization		20,413	20,230	40,807	40,341
Income from operations		12,606	12,914	86,678	65,915
Net Income		2,019	2,197	38,660	30,683
Capital expenditures		32,099	30,937	62,153	58,072
Assets		2,943,144	2,923,916	$ 2,943,144	2,923,916	$ 3,069,410
Operating Segments [Member]
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
Operating Segments [Member] | Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[1]	92,135	132,891	$ 342,239	389,197
Cost of Gas		20,871	62,176	129,282	187,881
Environmental Remediation Expense		1,893	0	6,922	0
Gross Profit		69,371	70,715	206,035	201,316
Depreciation and amortization		18,961	18,602	37,721	37,077
Income from operations		9,714	12,163	82,009	64,043
Net Income		507	2,245	36,359	30,580
Capital expenditures		31,295	30,464	60,472	56,273
Assets		2,663,817	2,638,569	2,663,817	2,638,569	2,792,736
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues		6,992	5,333	12,361	10,636
Depreciation and amortization		1,452	1,628	3,086	3,264
Income from operations		2,879	739	4,605	1,794
Net Income		1,439	(86)	2,175	28
Capital expenditures		804	473	1,681	1,799
Assets		263,498	270,434	263,498	270,434	261,750
Operating Segments [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues		56	56	112	112
Depreciation and amortization		0	0	0	0
Income from operations		13	12	64	78
Net Income		73	38	126	75
Capital expenditures		0	0	0	0
Assets		$ 15,829	$ 14,913	$ 15,829	$ 14,913	$ 14,924

[1] Utility operating revenues include environmental recovery revenues, which are collections received from customers through our environmental recovery mechanism in Oregon, offset by environmental remediation expense. Collections under this mechanism began in November 2015.