Risk Management and Hedging Activities Risk Management and Hedging Activities - Derivative Contract Volumes (Details) (Commodity Contract [Member])	Sep. 30, 2012 MWh	Dec. 31, 2011 MWh
Electricity sales (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	(2,000,000)	(2,000,000)
Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	73,000,000	96,000,000
Fuel oil purchases (in gallons) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	4,000,000	17,000,000