Segment Information (details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Abstract
Net operating revenues	$ 119,910,000	$ 47,783,000	$ 67,232,000	$ 134,508,000	$ 118,251,000	$ 46,211,000	$ 72,193,000	$ 130,926,000	$ 369,433,000	$ 367,581,000	$ 376,887,000
Depreciation and amortization									70,004,000	65,124,000	62,814,000
Income from operations									144,845,000	157,605,000	158,716,000
Net income	29,244,000	(8,312,000)	2,193,000	40,773,000	29,591,000	(7,420,000)	6,888,000	43,608,000	63,898,000	72,667,000	75,122,000
Total assets	2,746,574,000				2,616,616,000				2,746,574,000	2,616,616,000
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 percent								50 to 60 percent
Percent Of Total Local Gas Distribution Margin Attributed To Residential And Commercial Customers	80 to 90 percent								80 to 90 percent
Percent Of Total Local Gas Distribution Volume Attributed To Industrial Customers	40 to 50 percent								40 to 50 percent
Percent Of Total Local Gas Distribution Margin Attributed To Industrial Customers	5 to 15 percent								5 to 15 percent
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 Segment [Member]
Segment Reporting Abstract
Net operating revenues									342,970,000	346,148,000	357,005,000
Depreciation and amortization									63,843,000	62,661,000	61,472,000
Income from operations									135,722,000	145,688,000	142,228,000
Net income									60,527,000	66,262,000	65,960,000
Total assets	2,435,888,000				2,310,388,000				2,435,888,000	2,310,388,000
Gas Storage Segment [Member]
Segment Reporting Abstract
Net operating revenues									26,354,000	21,249,000	19,738,000
Depreciation and amortization									6,161,000	2,463,000	1,342,000
Income from operations									9,090,000	11,855,000	16,442,000
Net income									4,101,000	6,110,000	8,923,000
Total assets	294,637,000				282,945,000				294,637,000	282,945,000
Other Segment [Member]
Segment Reporting Abstract
Net operating revenues									109,000	184,000	144,000
Depreciation and amortization									0	0	0
Income from operations									33,000	62,000	46,000
Net income									(730,000)	295,000	239,000
Total assets	$ 16,049,000				$ 23,283,000				$ 16,049,000	$ 23,283,000