Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commodity Derivatives, Net [Member]
Commodity Derivatives [Abstract]
Number of commodity derivatives	0	0
Cash Flow Hedges [Member] | Interest Rate [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	$ 0.3	$ 0.3	$ 0.3
Credit Concentration Risk [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale 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 activities with credit exposure	3
Wholesale credit exposure for most significant counterparties	7.6
Percentage of wholesale credit exposure for most significant counterparties (in hundredths)	12.00%
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 activities with credit exposure	6
Wholesale credit exposure for most significant counterparties	34.0
Percentage of wholesale credit exposure for most significant counterparties (in hundredths)	55.00%
Credit Concentration Risk [Member] | Credit Quality Less Than Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale activities with credit exposure	1
Wholesale credit exposure for most significant counterparties	6.8
Percentage of wholesale credit exposure for most significant counterparties (in hundredths)	11.00%
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)