Segment Information (details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting [Abstract]
Operating revenues	$ 229,476,000	$ 87,501,000	$ 103,991,000	$ 309,639,000	$ 264,652,000	$ 90,916,000	$ 157,354,000	$ 315,133,000	$ 730,607,000	$ 828,055,000	$ 792,115,000
Depreciation and amortization									73,017,000	70,004,000	65,124,000
Income from operations									142,180,000	144,845,000	157,605,000
Net income	28,397,000	(10,558,000)	1,409,000	40,607,000	29,244,000	(8,312,000)	2,193,000	40,773,000	59,855,000	63,898,000	72,667,000
Capital expenditures									(132,029,000)	(100,534,000)	(248,505,000)
Total assets	2,818,753,000				2,746,574,000				2,818,753,000	2,746,574,000
Subsidiary Discussion [Abstract]
Percent Of Local Gas Distribution Customers In Oregon	90.00%								90.00%
Percent Of Local Gas Distribution Customers In Washington	10.00%								10.00%
Percent Of Total Local Gas Distribution Volume Attributed To Residential And Commercial Customers	50% to 60%								50% to 60%
Percent Of Total Local Gas Distribution Margin Attributed To Residential And Commercial Customers	80% to 90%								80% to 90%
Percent Of Total Local Gas Distribution Volume Attributed To Industrial Customers	40% to 50%								40% to 50%
Percent Of Total Local Gas Distribution Margin Attributed To Industrial Customers	5% to 15%								5% to 15%
Remaining Local Gas Margin Attributed To Misc	10.00%								10.00%
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	1,100,000				1,100,000				1,100,000	1,100,000
Utility Margin [Abstract]
Operating revenues	229,476,000	87,501,000	103,991,000	309,639,000	264,652,000	90,916,000	157,354,000	315,133,000	730,607,000	828,055,000	792,115,000
Cost of gas									355,335,000	458,508,000	424,494,000
Utility margin									344,527,000	342,970,000	346,148,000
Utility Segment [Member]
Segment Reporting [Abstract]
Operating revenues									699,862,000	801,478,000	770,642,000
Depreciation and amortization									66,545,000	63,843,000	62,661,000
Income from operations									128,854,000	135,722,000	145,688,000
Net income									55,125,000	60,527,000	66,262,000
Capital expenditures									(130,151,000)	(94,049,000)	(85,929,000)
Total assets	2,511,288,000				2,435,888,000				2,511,288,000	2,435,888,000
Utility Margin [Abstract]
Operating revenues									699,862,000	801,478,000	770,642,000
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Operating revenues									30,520,000	26,354,000	21,250,000
Depreciation and amortization									6,472,000	6,161,000	2,463,000
Income from operations									13,226,000	9,090,000	11,855,000
Net income									4,521,000	4,101,000	6,110,000
Capital expenditures									(1,541,000)	(6,485,000)	(161,634,000)
Total assets	291,568,000				294,637,000				291,568,000	294,637,000
Utility Margin [Abstract]
Operating revenues									30,520,000	26,354,000	21,250,000
Other Segment [Member]
Segment Reporting [Abstract]
Operating revenues									225,000	223,000	223,000
Depreciation and amortization									0	0	0
Income from operations									100,000	33,000	62,000
Net income									209,000	(730,000)	295,000
Capital expenditures									(337,000)	0	(942,000)
Total assets	15,897,000				16,049,000				15,897,000	16,049,000
Utility Margin [Abstract]
Operating revenues									$ 225,000	$ 223,000	$ 223,000