Risk Management and Hedging Activities Risk Management and Hedging Activities - Derivative Contract Volumes (Details) (Commodity Contract [Member])	Mar. 31, 2013 MWh	Dec. 31, 2012 MWh
Electricity sales (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	(1,000,000)	(1,000,000)
Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	67,000,000	74,000,000
Fuel oil purchases (in gallons) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	12,000,000	16,000,000