Income Taxes (Details) (USD $)	12 Months Ended				12 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2010 Provision	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2011 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 State Jurisdiction [Member]	Dec. 31, 2011 State Jurisdiction [Member]	Sep. 30, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 State Jurisdiction [Member]
Medicare Part D [Abstract]
Number of applicable provisions addressing deductibility of retiree health care costs				1
Tax Audits [Abstract]
Year(s) no longer subject to audit as statute of limitations has expired										2008
Earliest year subject to examination										2009	2008
Year(s) under examination									2010 and 2011		None
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit - Permanent tax positions	$ 200,000	$ 200,000
Unrecognized tax benefit - Temporary tax positions	3,700,000	4,600,000
Total unrecognized tax benefit	3,900,000	4,800,000	4,300,000
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at Jan. 1	4,800,000	4,300,000	2,900,000
Additions based on tax positions related to the current year	1,100,000	1,500,000	1,300,000
Reductions based on tax positions related to the current year	(1,600,000)	(200,000)	0
Additions for tax positions of prior years	800,000	2,500,000	200,000
Reductions for tax positions of prior years	(1,200,000)	(300,000)	(100,000)
Settlements with taxing authorities	0	(3,000,000)	0
Balance at Dec. 31	3,900,000	4,800,000	4,300,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards	(2,000,000)	(2,000,000)
Amounts accrued for penalties related to unrecognized tax benefits	0	0	0
Operating Loss and Tax Credit Carryforwards [Line Items]
NOL carryforward					158,000,000	175,100,000	21,000,000	37,300,000
Federal tax credit carryforwards					1,300,000	1,300,000
Carryforward expiration date range, low					2021		2014
Carryforward expiration date range, high					2031		2031
Effective Tax Rate Reconciliation [Abstract]
Federal statutory rate (in hundredths)	35.00%	35.00%	35.00%
Increases (Decreases) in Tax From [Abstract]
State income taxes, net of federal income tax benefit (in hundredths)	2.20%	2.90%	1.70%
Regulatory differences - utility plant items (in hundredths)	(0.40%)	(0.10%)	0.20%
Tax credits recognized (in hundredths)	(0.20%)	(0.30%)	(0.40%)
Medicare Part D (in hundredths)	0.00%	0.00%	1.50%
Other, net (in hundredths)	0.30%	0.50%	0.50%
Effective income tax rate (in hundredths)	36.90%	38.00%	38.50%
Components of Income Tax Expense [Abstract]
Current federal tax expense (benefit)	6,549,000	(1,993,000)	19,850,000
Current state tax expense	2,712,000	3,287,000	2,669,000
Current change in unrecognized tax (benefit) expense	(824,000)	(1,394,000)	1,376,000
Deferred federal tax expense	50,189,000	50,903,000	25,905,000
Deferred state tax expense	3,069,000	3,240,000	747,000
Deferred change in unrecognized tax expense (benefit)	861,000	1,365,000	(1,340,000)
Deferred investment tax credits	(327,000)	(275,000)	(341,000)
Total income tax expense	62,229,000	55,133,000	48,866,000
Components of Deferred Income Tax Expense [Abstract]
Deferred tax expense excluding items below	55,749,000	56,181,000	25,318,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(1,533,000)	(575,000)	90,000
Tax expense allocated to other comprehensive income	(97,000)	(98,000)	(96,000)
Deferred tax expense	54,119,000	55,508,000	25,312,000
Deferred Tax Liabilities [Abstract]
Differences between book and tax bases of property	654,259,000	597,122,000
Employee benefits	51,394,000	56,069,000
Other	24,034,000	21,359,000
Total deferred tax liabilities	729,687,000	674,550,000
Deferred Tax Assets [Abstract]
NOL carryforward	59,562,000	65,278,000
Deferred fuel costs	11,952,000	2,473,000
Unbilled revenue - fuel costs	10,555,000	13,138,000
Regulatory liabilities	6,176,000	7,501,000
Other	7,359,000	7,826,000
Total deferred tax assets	95,604,000	96,216,000
Net deferred tax liability	$ 634,083,000	$ 578,334,000