Risk Management and Hedging Activities - Derivative Contract Volumes (Details) - Commodity Contract [Member] gal in Millions, MWh in Millions, Dth in Millions	Sep. 30, 2015 MWh gal Dth	Dec. 31, 2014 MWh gal Dth
Electricity sales (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | MWh	(1)	(1)
Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount	111	113
Fuel oil purchases (in gallons) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Nonmonetary Notional Amount | gal	3	3