Commitments and Contingencies (Details) (USD $) In Millions, except Per Share data, unless otherwise specified		12 Months Ended			12 Months Ended				117 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Capital Expenditures [Member]	Dec. 31, 2012 Manufactured Gas Plant Sites [Member]	Dec. 31, 2011 Manufactured Gas Plant Sites [Member]	Dec. 31, 2012 Coal combustion product landfill sites [Member]	Dec. 31, 2011 Coal combustion product landfill sites [Member]	Dec. 31, 2010 Coal combustion product landfill sites [Member]	Dec. 31, 2012 EPA - Consent Decree [Member]	Dec. 31, 2012 EPA - Consent Decree [Member]	Dec. 08, 2011 Valley Power Plant [Member] T	Dec. 31, 2011 Cash Balance Pension Plan [Member]	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]
Commitments and Contingencies [Line Items]
VAPP Conversion Cost												$ 60	$ 65
Commitment and Contingencies (Textuals) [Abstract]
Estimated total capital expenditures commitment amount		692.7
Reserves related to future remediation costs			38.2	37.5
Expense related to environmental remediation					0.3	0.2	0.4		1,200.0
Reserves related to landfill sites					0
Loss contingency, settlement agreement, terms											As part of the settlement agreement, we agreed to class certification for all similarly situated plaintiffs.
Future minimum payments for the next five years and thereafter for our operating lease contracts
2013	6.5
2014	3.9
2015	3.9
2016	3.7
2017	3.2
Thereafter	25.9
Total	47.1
Commitments and Contingencies (Textuals) [Abstract]
Estimated future detailed site investigation and remediation costs, minimum			16
Estimated Future detailed site investigation and remediation costs, maximum			62
Period for future costs of detailed site investigation and remediation costs			over the next ten years
Expense related to environmental remediation					0.3	0.2	0.4		1,200.0
Estimated Future Environmental Remediation Expense								22
Period to recover remediation cost of manufactured gas plant sites			5
Loss Contingency, Settlement Agreement, Consideration											$ 45.0
Loss Contingency Range Of Possible Loss, Per Share, Maximum											$ 0.04
Target Annual Sulfur Dioxide Emissions Threshold										4,500
Target Reduction in Sulfur Dioxide Emissions, As a Percent										65.00%