Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended					12 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2010 Provision	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	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		$ 100,000	$ 0
Unrecognized tax benefit - Temporary tax positions		1,200,000	1,500,000
Total unrecognized tax benefit		1,300,000	1,500,000	1,900,000
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at Jan. 1	1,200,000	1,500,000	1,900,000	1,200,000
Additions based on tax positions related to the current year		500,000	600,000	700,000
Reductions based on tax positions related to the current year		(200,000)	(100,000)	0
Additions for tax positions of prior years		300,000	700,000	100,000
Reductions for tax positions of prior years		(800,000)	(300,000)	(100,000)
Settlements with taxing authorities		0	(1,200,000)	0
Lapse of applicable statutes of limitations.		0	(100,000)	0
Balance at Dec. 31		1,300,000	1,500,000	1,900,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards		(900,000)	(1,100,000)
Amounts accrued for penalties related to unrecognized tax benefits		0	0	0
Operating Loss and Tax Credit Carryforwards [Line Items]
NOL carryforward					44,300,000	59,300,000	3,400,000	3,500,000
Federal tax credit carryforwards					8,000,000	8,000,000
Carryforward expiration date range, low					2021
Carryforward expiration date range, high					2032
Carryforward expiration date							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)		3.40%	4.40%	3.50%
Change in unrecognized tax benefits (in hundredths)		0.10%	(0.20%)	0.00%
Tax credits recognized (in hundredths)		(0.90%)	(0.90%)	(1.10%)
Regulatory differences - utility plant items (in hundredths)		(0.30%)	0.50%	(0.70%)
Medicare Part D (in hundredths)		0.00%	0.00%	1.00%
Other, net (in hundredths)		(0.10%)	0.90%	0.20%
Effective income tax rate from continuing operations (in hundredths)		37.20%	39.70%	37.90%
Components of Income Tax Expense [Abstract]
Current federal tax expense (benefit)		930,000	(1,540,000)	3,754,000
Current state tax expense		2,216,000	1,573,000	1,536,000
Current change in unrecognized tax (benefit) expense		(69,000)	(1,418,000)	730,000
Deferred federal tax expense		25,089,000	30,251,000	19,133,000
Deferred state tax expense		1,890,000	4,105,000	2,288,000
Deferred change in unrecognized tax expense (benefit)		128,000	1,254,000	(707,000)
Deferred investment tax credits		(626,000)	(611,000)	(622,000)
Total income tax expense		29,558,000	33,614,000	26,112,000
Components of Deferred Income Tax Expense [Abstract]
Deferred tax expense excluding items below		27,995,000	34,017,000	20,987,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(837,000)	1,644,000	(222,000)
Tax expense allocated to other comprehensive income		(51,000)	(51,000)	(51,000)
Deferred tax expense		27,107,000	35,610,000	20,714,000
Deferred Tax Liabilities [Abstract]
Differences between book and tax bases of property		265,202,000	241,003,000
Regulatory assets		52,898,000	52,131,000
Employee benefits		16,862,000	21,013,000
Other		10,535,000	9,328,000
Total deferred tax liabilities		345,497,000	323,475,000
Deferred Tax Assets [Abstract]
Environmental remediation		43,344,000	44,154,000
NOL carryforward		17,588,000	22,318,000
Tax credit carryforward		8,011,000	7,998,000
Regulatory liabilities		5,927,000	6,222,000
Deferred investment tax credits		5,766,000	5,705,000
Other		2,191,000	2,403,000
Total deferred tax assets		82,827,000	88,800,000
Net deferred tax liability		$ 262,670,000	$ 234,675,000