Risk Management and Hedging Activities - Derivative Contract Volumes (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014 MWh	Dec. 31, 2013 MWh
Electricity sales (in megawatt hours) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	(8,000,000)	(5,000,000)
Natural gas purchases (in decatherms) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	334,000,000	322,000,000
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	6,000,000	9,000,000
Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ 650	$ 650
Mortgage Sale Commitments, Net [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, Notional Amount	$ (60)	$ (121)