Risk Management and Hedging Activities - Derivative Contract Volumes (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Interest rate swap [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ 458	$ 470
Electricity sales (in megawatt hours) [Member] | Commodity derivative [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] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	172,000,000	130,000,000
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	12,000,000	16,000,000