Segment Information (details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting [Abstract]
Operating revenues	$ 260,748,000	$ 88,195,000	$ 131,714,000	$ 277,861,000	$ 229,476,000		$ 87,501,000		$ 103,991,000		$ 309,639,000						$ 758,518,000	$ 730,607,000	$ 828,055,000
Depreciation and amortization																	75,905,000	73,017,000	70,004,000
Income from operations																	142,746,000	142,180,000	144,845,000
Net income	29,006,000	(8,233,000)	2,126,000	37,639,000	28,147,000	[1]	(10,879,000)	[1]	1,227,000	[1]	40,284,000	[1]	28,766,000	(8,527,000)	1,985,000	40,820,000	60,538,000	58,779,000	63,044,000
Capital expenditures																	(138,924,000)	(132,029,000)	(100,534,000)
Total assets	2,970,911,000				2,813,120,000		2,685,148,000		2,630,450,000		2,722,873,000		2,742,718,000	2,564,775,000	2,519,284,000	2,569,186,000	2,970,911,000	2,813,120,000	2,742,718,000
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																	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	1,200,000				1,100,000												1,200,000	1,100,000
Utility Margin [Abstract]
Operating revenues	260,748,000	88,195,000	131,714,000	277,861,000	229,476,000		87,501,000		103,991,000		309,639,000						758,518,000	730,607,000	828,055,000
Cost of gas																	373,298,000	355,335,000	458,508,000
Utility margin																	353,884,000	344,527,000	342,970,000
Utility Segment [Member]
Segment Reporting [Abstract]
Operating revenues																	727,182,000	699,862,000	801,478,000
Depreciation and amortization																	69,420,000	66,545,000	63,843,000
Income from operations																	128,066,000	128,854,000	135,722,000
Net income																	54,920,000	54,049,000	59,673,000
Capital expenditures																	(137,466,000)	(130,151,000)	(94,049,000)
Total assets	2,644,367,000				2,505,655,000												2,644,367,000	2,505,655,000
Utility Margin [Abstract]
Operating revenues																	727,182,000	699,862,000	801,478,000
Gas Storage Segment [Member]
Segment Reporting [Abstract]
Operating revenues																	31,112,000	30,520,000	26,354,000
Depreciation and amortization																	6,485,000	6,472,000	6,161,000
Income from operations																	14,669,000	13,226,000	9,090,000
Net income																	5,569,000	4,521,000	4,101,000
Capital expenditures																	(1,458,000)	(1,541,000)	(6,485,000)
Total assets	310,097,000				291,568,000												310,097,000	291,568,000
Utility Margin [Abstract]
Operating revenues																	31,112,000	30,520,000	26,354,000
Other Segments [Member]
Segment Reporting [Abstract]
Operating revenues																	224,000	225,000	223,000
Depreciation and amortization																	0	0	0
Income from operations																	11,000	100,000	33,000
Net income																	49,000	209,000	(730,000)
Capital expenditures																	0	(337,000)	0
Total assets	16,447,000				15,897,000												16,447,000	15,897,000
Utility Margin [Abstract]
Operating revenues																	$ 224,000	$ 225,000	$ 223,000

[1] Prior period balances have been adjusted for a prior period error identified during the first quarter of 2013. See Note 16 for reconciliation to amounts previously reported.