Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting [Abstract]
Operating revenues	$ 240,283,000	$ 87,199,000	$ 133,169,000	$ 293,386,000	$ 260,748,000	$ 88,195,000	$ 131,714,000	$ 277,861,000	$ 754,037,000	$ 758,518,000	$ 730,607,000
Depreciation and amortization									79,193,000	75,905,000	73,017,000
Income from operations									142,965,000	142,746,000	142,180,000
Net income	28,470,000	(8,733,000)	1,071,000	37,884,000	29,006,000	(8,233,000)	2,126,000	37,639,000	58,692,000	60,538,000	58,779,000
Capital expenditures									(120,092,000)	(138,924,000)	(132,029,000)
Total assets	3,064,945,000				2,970,911,000				3,064,945,000	2,970,911,000	2,813,120,000
Subsidiary Discussion [Abstract]
Percent Of Local Gas Distribution Customers In Oregon	89.00%								89.00%
Percent Of Local Gas Distribution Customers In Washington	11.00%								11.00%
Percent Of Total Local Gas Distribution Volume Attributed To Residential And Commercial Customers									around 60%
Percent Of Total Local Gas Distribution Margin Attributed To Residential And Commercial Customers									90%
Segment Reporting, Disclosure of Major Customers									over 10%
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%
Percent Ownership In Joint Project	75.00%								75.00%
Precent Interest In Subsidiary	10.00%								10.00%
Total Assets At NNG Financial	800,000				1,200,000				800,000	1,200,000
Utility Margin [Abstract]
Operating revenues	240,283,000	87,199,000	133,169,000	293,386,000	260,748,000	88,195,000	131,714,000	277,861,000	754,037,000	758,518,000	730,607,000
Cost of gas									365,490,000	373,298,000	355,335,000
Utility margin									366,088,000	353,884,000	344,527,000
Utility Segment [Member]
Segment Reporting [Abstract]
Operating revenues									731,578,000	727,182,000	699,862,000
Depreciation and amortization									72,660,000	69,420,000	66,545,000
Income from operations									138,711,000	128,066,000	128,854,000
Net income									58,587,000	54,920,000	54,049,000
Capital expenditures									(117,322,000)	(137,466,000)	(130,151,000)
Total assets	2,775,011,000				2,644,367,000				2,775,011,000	2,644,367,000	2,505,655,000
Utility Margin [Abstract]
Operating revenues									731,578,000	727,182,000	699,862,000
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Operating revenues									22,235,000	31,112,000	30,520,000
Depreciation and amortization									6,533,000	6,485,000	6,472,000
Income from operations									3,987,000	14,669,000	13,226,000
Net income									(364,000)	5,569,000	4,521,000
Capital expenditures									(2,770,000)	(1,458,000)	(1,541,000)
Total assets	273,813,000				310,097,000				273,813,000	310,097,000	291,568,000
Utility Margin [Abstract]
Operating revenues									22,235,000	31,112,000	30,520,000
Other Segments [Member]
Segment Reporting [Abstract]
Operating revenues									224,000	224,000	225,000
Depreciation and amortization									0	0	0
Income from operations									267,000	11,000	100,000
Net income									469,000	49,000	209,000
Capital expenditures									0	0	(337,000)
Total assets	16,121,000				16,447,000				16,121,000	16,447,000	15,897,000
Utility Margin [Abstract]
Operating revenues									$ 224,000	$ 224,000	$ 225,000