Segment Information (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Operating revenues		$ 255,529	$ 261,665
Cost of Gas		108,411	125,705
Environmental Remediation Expense		5,029	0
Depreciation and amortization		20,394	20,111
Income from operations		74,072	53,001
Net Income		36,641	28,486
Capital expenditures		30,054	27,135
Assets		$ 3,002,033	2,974,112	$ 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]	$ 250,104	256,306
Cost of Gas		108,411	125,705
Environmental Remediation Expense		5,029	0
Gross Profit		136,664	130,601
Depreciation and amortization		18,760	18,475
Income from operations		72,295	51,880
Net Income		35,852	28,335
Capital expenditures		29,177	25,809
Assets		2,726,696	2,688,304	2,792,736
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues		5,369	5,303
Depreciation and amortization		1,634	1,636
Income from operations		1,726	1,055
Net Income		736	114
Capital expenditures		877	1,326
Assets		260,535	270,905	261,750
Operating Segments [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues		56	56
Depreciation and amortization		0	0
Income from operations		51	66
Net Income		53	37
Capital expenditures		0	0
Assets		$ 14,802	$ 14,903	$ 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.