Segment Information (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Operating revenues		$ 297,323	$ 255,529
Cost of Gas		143,611	108,411
Environmental Remediation Expense		6,954	5,029
Depreciation and amortization		21,085	20,394
Income from operations		76,228	74,072
Net Income		40,310	36,641
Capital expenditures		38,924	30,054
Assets		$ 3,070,656	3,002,033	$ 3,079,801
Operating Segments [Member]
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		2
Operating Segments [Member] | Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[1]	$ 292,726	250,104
Cost of Gas		143,611	108,411
Environmental Remediation Expense		6,954	5,029
Gross Profit		142,161	136,664
Depreciation and amortization		19,624	18,760
Income from operations		75,823	72,295
Net Income		40,192	35,852
Capital expenditures		38,854	29,177
Assets		2,799,638	2,726,696	2,806,627
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues		4,541	5,369
Depreciation and amortization		1,461	1,634
Income from operations		606	1,726
Net Income		61	736
Capital expenditures		70	877
Assets		254,260	260,535	256,333
Corporate, Non-Segment [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues		56	56
Depreciation and amortization		0	0
Income from operations		(201)	51
Net Income		57	53
Capital expenditures		0	0
Assets		$ 16,758	$ 14,802	$ 16,841

[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.