Revision (Details) (USD $)	3 Months Ended																				12 Months Ended			3 Months Ended								12 Months Ended				120 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	Dec. 31, 2012 Scenario, Previously Reported	Sep. 30, 2012 Scenario, Previously Reported	Jun. 30, 2012 Scenario, Previously Reported	Mar. 31, 2012 Scenario, Previously Reported	Dec. 31, 2011 Scenario, Previously Reported	Sep. 30, 2011 Scenario, Previously Reported	Jun. 30, 2011 Scenario, Previously Reported	Mar. 31, 2011 Scenario, Previously Reported	Dec. 31, 2012 Scenario, Previously Reported	Dec. 31, 2011 Scenario, Previously Reported	Dec. 31, 2012 Restatement Adjustment	Dec. 31, 2011 Restatement Adjustment	Dec. 31, 2012 Restatement Adjustment	Dec. 31, 2010 Restatement Adjustment
Pre-tax decrease to regulatory assets and other income and expense																																				$ 5,600,000
Income Statement [Abstract]
Other income and expense, net									887,000		1,180,000		620,000		472,000		(384,000)	1,426,000	779,000	1,291,000	4,669,000	3,159,000	3,112,000	1,300,000	1,710,000	921,000	1,005,000	406,000	1,781,000	1,122,000	1,214,000	4,936,000	4,523,000	(1,777,000)	(1,411,000)
Income before income taxes									46,364,000		(14,124,000)		1,995,000		67,947,000		48,366,000	(14,367,000)	3,166,000	68,704,000	102,243,000	102,182,000	105,869,000	46,777,000	(13,594,000)	2,296,000	68,480,000	49,156,000	(14,012,000)	3,509,000	68,627,000	103,959,000	107,280,000	(1,777,000)	(1,411,000)
Income tax expense									18,217,000		(3,245,000)		768,000		27,663,000		19,600,000	(5,840,000)	1,181,000	27,884,000	41,705,000	43,403,000	42,825,000	18,380,000	(3,036,000)	887,000	27,873,000	19,912,000	(5,700,000)	1,316,000	27,854,000	44,104,000	43,382,000	(701,000)	(557,000)
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	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	(1,076,000)	(854,000)
Comprehensive income									26,157,000		(10,712,000)		1,393,000		40,450,000		27,132,000	(8,381,000)	2,131,000	40,966,000	63,471,000	57,288,000	61,848,000	26,407,000	(10,391,000)	1,575,000	40,773,000	27,610,000	(8,166,000)	2,339,000	40,919,000	58,364,000	62,702,000	(1,076,000)	(854,000)
Earnings per share of common stock - basic	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.05	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.08	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.19	$ 2.36	$ 1.06	$ (0.39)	$ 0.05	$ 1.52	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 2.23	$ 2.39	$ (0.04)	$ (0.03)
Earnings per share of common stock - diluted	$ 1.07	[2]	$ (0.31)	[2]	$ 0.08	[2]	$ 1.40	[2]	$ 1.04	[1],[2]	$ (0.41)	[1],[2]	$ 0.05	[1],[2]	$ 1.50	[1],[2]	$ 1.07	$ (0.32)	$ 0.07	$ 1.53	$ 2.24	$ 2.18	$ 2.36	$ 1.05	$ (0.39)	$ 0.05	$ 1.51	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 2.22	$ 2.39	$ (0.04)	$ (0.03)
Assets, Noncurrent [Abstract]
Regulatory assets	369,603,000								382,255,000		362,472,000		362,290,000		364,132,000		367,536,000	325,692,000	323,371,000	343,085,000	369,603,000	382,255,000	367,536,000	387,888,000	367,692,000	366,981,000	368,521,000	371,392,000	328,757,000	326,081,000	345,452,000	387,888,000	371,392,000	(5,633,000)	(3,856,000)	(5,633,000)
Total non-current assets	2,640,463,000								2,529,421,000		2,487,247,000		2,443,668,000		2,411,983,000		2,394,029,000	2,314,228,000	2,291,390,000	2,288,481,000	2,640,463,000	2,529,421,000	2,394,029,000	2,535,054,000	2,492,467,000	2,448,359,000	2,416,372,000	2,397,885,000	2,317,293,000	2,294,100,000	2,290,848,000	2,535,054,000	2,397,885,000	(5,633,000)	(3,856,000)	(5,633,000)
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	2,818,753,000	2,690,368,000	2,635,141,000	2,727,262,000	2,746,574,000	2,567,840,000	2,521,994,000	2,571,553,000	2,818,753,000	2,746,574,000	(5,633,000)	(3,856,000)	(5,633,000)
Liabilities, Noncurrent [Abstract]
Deferred tax liabilities	532,036,000								444,377,000		428,821,000		438,217,000		436,750,000		411,683,000	393,003,000	397,751,000	395,419,000	532,036,000	444,377,000	411,683,000	446,604,000	430,885,000	440,073,000	438,486,000	413,209,000	394,217,000	398,825,000	396,357,000	446,604,000	413,209,000	(2,227,000)	(1,526,000)	(2,227,000)
Total deferred credits and other non-current liabilities	1,104,548,000								1,023,357,000		983,665,000		989,151,000		997,292,000		974,396,000	865,713,000	873,768,000	872,776,000	1,104,548,000	1,023,357,000	974,396,000	1,025,584,000	985,729,000	991,007,000	999,028,000	975,922,000	866,927,000	874,842,000	873,714,000	1,025,584,000	975,922,000	(2,227,000)	(1,526,000)	(2,227,000)
Equity [Abstract]
Retained earnings	393,681,000								382,347,000		366,428,000		389,247,000		399,946,000		371,575,000	354,723,000	374,853,000	384,470,000	393,681,000	382,347,000	371,575,000	385,753,000	369,584,000	392,082,000	402,599,000	373,905,000	356,574,000	376,489,000	385,899,000	385,753,000	373,905,000	(3,406,000)	(2,330,000)	(3,406,000)	1,400,000
Total equity	751,872,000								729,627,000		714,403,000		734,735,000		743,318,000		712,158,000	694,754,000	712,992,000	721,799,000	751,872,000	729,627,000	712,158,000	733,033,000	717,559,000	737,570,000	745,971,000	714,488,000	696,605,000	714,628,000	723,228,000	733,033,000	714,488,000	(3,406,000)	(2,330,000)	(3,406,000)
Total liabilities and equity	$ 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	$ 2,818,753,000	$ 2,690,368,000	$ 2,635,141,000	$ 2,727,262,000	$ 2,746,574,000	$ 2,567,840,000	$ 2,521,994,000	$ 2,571,553,000	$ 2,818,753,000	$ 2,746,574,000	$ (5,633,000)	$ (3,856,000)	$ (5,633,000)

[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.

[2] Quarterly earnings (loss) per share are based upon the average number of common shares outstanding during each quarter. Variations in earnings between quarterly periods are due primarily to the seasonal nature of our business.