Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of the federal statutory rate to the effective income tax rate [Abstract]
Statutory rate: (in hundredths)	35.00%	35.00%	35.00%
State and local taxes-net of federal benefit (in hundredths)	4.20%	3.80%	2.30%
Amortization of investment tax credit (in hundredths)	(0.30%)	(0.40%)	(0.50%)
Depletion (in hundredths)	(1.90%)	(2.00%)	(2.00%)
Other tax credits (in hundredths)	(0.20%)	(0.20%)	(0.20%)
Adjustment of income tax accruals and all other-net (in hundredths)	1.10%	(0.40%)	(2.10%)
Effective tax rate (in hundredths)	37.90%	35.80%	32.50%
Noncurrent deferred tax liabilities (assets) [Abstract]
Depreciation and cost recovery timing differences	$ 625.5	$ 565.7
Leveraged leases	13.8	14.2
Regulatory assets recoverable through future rates	25.1	20.1
Other comprehensive income	(10.2)	(4.2)
Alternative minimum tax carryforward - long term	(35.1)	(48.6)
Employee benefit obligations	(9.4)	(18.9)
Net operating loss and other carryforwards	(6.7)	(3.8)
Regulatory liabilities to be settled through future rates	(17.2)	(4.8)
Impairments	(11.4)	(4.4)
Other - net	1.3	0
Deferred Tax Liabilities, Noncurrent	575.7	515.3
Deferred Tax (Assets) Liabilities Current [Abstract]
Deferred fuel costs-net	6.0	2.4
Demand side management programs	0.7	2.5
Alternative Minimum Tax Carryforward - Short Term	(15.6)	(0.8)
Other - net	(7.1)	(7.9)
Net current deferred tax asset	(16.0)	(3.8)
Net deferred tax liability	559.7	511.5
Investment tax credits	4.3	5.0
Remaining life of NOL's and business credit carryforwards (in years)	5 to 20 years
Current: [Abstract]
Federal	4.4	(0.8)	(21.4)
State	10.3	6.2	0.6
Total current taxes	14.7	5.4	(20.8)
Deferred: [Abstract]
Federal	66.0	65.6	78.7
State	6.4	4.5	7.3
Total deferred taxes	72.4	70.1	86.0
Amortization of investment tax credits	(0.7)	(0.8)	(1.1)
Total income tax expense	86.4	74.7	64.1
Uncertain tax positions [Roll Forward]
Unrecognized tax benefits at beginning of period	13.3	11.5	2.2
Gross increases - tax positions in prior periods	3.3	1.6	1.1
Gross decreases - tax positions in prior periods	(4.5)	(0.3)	(1.8)
Gross increases - current period tax positions	0.6	1.0	9.0
Settlements	(0.3)	0	(0.1)
Lapse of statute of limitations	0	(0.5)	1.1
Unrecognized tax benefits at end of period	12.4	13.3	11.5
Uncertain tax positions [Abstract]
Amount of unrecognized tax benefits, which if recognized, that would impact the effective tax rate	0.7	0.7	0.5
Interest and penalties	0.4	0.3	0.2
Payment of interest and penalties accrued	1.3	0.9
Net liability for unrecognized tax benefits inclusive of interest, penalties and net of secondary impacts which are a component of the deferred income taxes and are benefits	$ 10.1	$ 9.8