Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues		$ 233,528	$ 87,727	$ 99,183	$ 255,529	$ 230,718	$ 93,128	$ 138,280	$ 261,665	$ 675,967	$ 723,791	$ 754,037
Cost of Gas										260,588	327,305	365,490
Environmental Remediation Expense										13,298	3,513	0
Depreciation and amortization										82,289	80,923	79,193
Income from operations										139,280	124,248	142,965
Net Income		28,275	$ (8,040)	$ 2,019	$ 36,641	29,705	$ (6,685)	$ 2,197	$ 28,486	58,895	53,703	58,692
Capital expenditures										139,511	118,320	120,092
Assets		3,079,801				3,069,410				$ 3,079,801	3,069,410	3,056,326
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]									$ 650,477	702,210	731,578
Cost of Gas										260,588	327,305	365,490
Environmental Remediation Expense										13,298	3,513	0
Gross Profit										376,591	371,392	366,088
Depreciation and amortization										76,289	74,410	72,660
Income from operations										130,570	119,215	138,711
Net Income										54,567	53,391	58,587
Capital expenditures										138,074	115,272	117,322
Assets		2,806,627				2,791,623				2,806,627	2,791,623	2,766,493
Operating Segments [Member] | Gas Storage Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues										25,266	21,356	22,235
Depreciation and amortization										6,000	6,513	6,533
Income from operations										9,136	5,032	3,987
Net Income										4,303	174	(364)
Capital expenditures										1,437	3,048	2,770
Assets		256,333				261,750				256,333	261,750	273,712
Corporate, Non-Segment [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues										224	225	224
Depreciation and amortization										0	0	0
Income from operations										(426)	1	267
Net Income										25	138	469
Capital expenditures										0	0	0
Assets		$ 16,841				16,037				$ 16,841	16,037	$ 16,121
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		$ 500				$ 700				$ 500	$ 700
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. Collections under this mechanism began in November 2015.