Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 Commodity Derivatives, Net [Member] Contract	Dec. 31, 2013 Credit Concentration Risk [Member] Counterparty	Dec. 31, 2013 Credit Concentration Risk [Member] Investment Grade Ratings from Standard & Poor's, Moody's, or Fitch Ratings [Member] Counterparty	Dec. 31, 2013 Credit Concentration Risk [Member] No Investment Grade Ratings from External Credit Rating Agencies [Member] Counterparty	Dec. 31, 2013 Credit Concentration Risk [Member] Credit Quality Less Than Investment Grade [Member] Counterparty	Dec. 31, 2013 Interest Rate Swap [Member]	Dec. 31, 2013 Electric Commodity [Member] MWh
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount							5,989,000	[1]
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)
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale activities with credit exposure		10	2	7	1
Wholesale credit exposure for most significant counterparties			$ 13.5	$ 37.4	$ 3.3
Percentage of wholesale credit exposure for most significant counterparties (in hundredths)			18.00%	51.00%	5.00%
Commodity Derivatives [Abstract]
Number of commodity derivatives	0

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