Commitments and Contingencies - Environmental Remediation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Gas plant
Loss Contingencies [Line Items]
Environmental remediation costs, low end estimate	$ 39.1
Environmental remediation costs, high end estimate	57.3
Accrual for environmental loss contingencies	39.1
Environmental expense and liabilities incurred	5.5
Electric
Loss Contingencies [Line Items]
Environmental remediation costs, low end estimate	8.2
Environmental remediation costs, high end estimate	27.9
Accrual for environmental loss contingencies	8.2
Environmental expense and liabilities incurred	$ 9.6

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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High-end estimate of the reasonably possible loss exposure in excess of the amount accrued for remediation, site restoration, postclosure, monitoring commitments, or other exit costs associated with the sale, disposal or abandonment of a particular property resulting from unanticipated contamination of the assets.

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- Definition

Low-end estimate of the reasonably possible loss exposure in excess of the amount accrued for remediation, site restoration, postclosure, monitoring commitments, or other exit costs associated with the sale, disposal or abandonment of a particular property resulting from unanticipated contamination of the assets.

+ References+ Details