Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues		$ 230,718	$ 93,128	$ 138,280	$ 261,665	$ 240,283	$ 87,199	$ 133,169	$ 293,386	$ 723,791	$ 754,037	$ 758,518
Cost of Gas										327,305	365,490	373,298
Environmental Remediation Expense										3,513	0	0
Depreciation and amortization										80,923	79,193	75,905
Income from operations										124,248	142,965	142,746
Net Income		29,705	$ (6,685)	$ 2,197	$ 28,486	28,470	$ (8,733)	$ 1,071	$ 37,884	53,703	58,692	60,538
Capital expenditures										118,320	120,092	138,924
Assets		3,076,692				3,064,945				$ 3,076,692	3,064,945	2,970,911
Operating Segments [Member]
Segment Reporting Information [Line Items]
Number of Reportable Segments										2
Segment Reporting, Disclosure of Major Customers										0
Operating Segments [Member] | Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[1]									$ 702,210	731,578	727,182
Cost of Gas										327,305	365,490	373,298
Environmental Remediation Expense										3,513	0	0
Gross Profit										371,392	366,088	353,884
Depreciation and amortization										74,410	72,660	69,420
Income from operations										119,215	138,711	128,066
Net Income										53,391	58,587	54,920
Capital expenditures										115,272	117,322	137,466
Assets		2,800,018				2,775,011				2,800,018	2,775,011	2,644,367
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues										21,356	22,235	31,112
Depreciation and amortization										6,513	6,533	6,485
Income from operations										5,032	3,987	14,669
Net Income										174	(364)	5,569
Capital expenditures										3,048	2,770	1,458
Assets		261,750				273,813				261,750	273,813	310,097
Operating Segments [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues										225	224	224
Depreciation and amortization										0	0	0
Income from operations										1	267	11
Net Income										138	469	49
Capital expenditures										0	0	0
Assets		$ 14,924				16,121				$ 14,924	16,121	$ 16,447
Mist Gas Storage Facility Member [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are Not In Utility Rates		80.00%								80.00%
Percent Of Pre Tax Income From Gas Storage Retained When The Costs Of The Capacity Are In Utility Rates		33.00%								33.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account		20.00%								20.00%
Percent Of Pre Tax Income From Gas Storage Credited To Deferred Regulatory Account When In Rates		67.00%								67.00%
Gill Ranch Gas Storage Facility Member [Member]
Segment Reporting Information [Line Items]
Percent Ownership In Joint Project		75.00%								75.00%
NNG Financial [Member]
Segment Reporting Information [Line Items]
Percent Interest In Pipeline		10.00%								10.00%
Assets		$ 700				$ 800				$ 700	$ 800
Customer Total [Member] | Geographic Concentration Risk [Member] | OREGON | Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage										89.00%
Customer Total [Member] | Geographic Concentration Risk [Member] | WASHINGTON | Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage										11.00%
Gross Profit Margin [Member] | Residential and Commerical Customers [Member] | Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage										90.00%
Volumes Delivered [Member] | Residential and Commerical Customers [Member] | Utility Segment [Member]
Segment Reporting Information [Line Items]
Concentration Risk, Percentage										60.00%

[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.