Risk Management and Hedging Activities - Derivative Contract Volumes (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015 MWh	Dec. 31, 2014 MWh
Electricity sales (in megawatt hours) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	9,000,000	6,000,000
Natural gas purchases (in decatherms) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	352,000,000	308,000,000
Fuel purchases (in gallons) [Member] | Commodity derivative [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount	10,000,000	2,000,000
Interest rate swap [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	443	443
Mortgage sale commitments, net [Member] | Interest Rate Contract [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, notional amount	(341)	(264)