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 and transmission 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 and transmission activities with credit exposure	3
Wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties	$ 16.4
Percentage of wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties (in hundredths)	17.00%
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 and transmission activities with credit exposure	7
Wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties	46.6
Percentage of wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties (in hundredths)	48.00%
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	$ (0.2)
Electric Commodity [Member]
Gross Notional Amounts of Commodity FTRs [Abstract]
Notional Amount	3,989,000	[1]	5,989,000	[1]

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