Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014 MWh		Dec. 31, 2013 MWh
Credit Concentration Risk [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	10
Credit Concentration Risk [Member] | Investment Grade Ratings from Standard & Poor's, Moody's, or Fitch Ratings [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	$ 26.6
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	27.00%
Credit Concentration Risk [Member] | No Investment Grade Ratings from External Credit Rating Agencies [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	43.6
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)	45.00%
Interest Rate Swap [Member]
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months	$ 0.5
Electric Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	83,000	[1],[2]	326,000	[1],[2]
Natural Gas Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	0	[1],[2]	6,398,000	[1],[2]
Vehicle Fuel Commodity [Member]
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount	194,000	[1],[2]	217,000	[1],[2]

[1] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

[2] Amounts are not reflective of net positions in the underlying commodities.