Commitments and Contingencies (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended			12 Months Ended
	Mar. 31, 2012	Mar. 31, 2012 Manufactured Gas Plant Sites [Member]	Mar. 31, 2012 Valley Power Plant [Member] T	Dec. 31, 2011 Cash Balance Pension Plan [Member]
Commitments and Contingencies [Line Items]
Loss Contingency, Range of Possible Loss, Minimum		$ 21
Loss Contingency, Range of Possible Loss, Maximum		65
Period for future costs of detailed site investigation and remediation costs		over the next ten years
Accrual for Environmental Loss Contingencies, Provision for New Losses		37.5
Target Annual Sulfur Dioxide Emissions Threshold			4,500
Target Reduction in Sulfur Dioxide Emissions, As a Percent			65.00%
Loss Contingency, Settlement Agreement, Consideration				In November 2011, we entered into a settlement agreement with the plaintiffs for $45.0 million, and the court promptly issued an order preliminarily approving the settlement. As part of the settlement agreement, we agreed to class certification for all similarly situated plaintiffs.
Loss Contingency Range Of Possible Loss, Per Share, Maximum				$ 0.04
Unrecognized Tax Benefits, Period Increase (Decrease)	$ (7.4)