Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities, Counterparty Credit Risk (Details) (Credit Concentration Risk, USD $) In Millions, unless otherwise specified	Sep. 30, 2014 Counterparty
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	8
Investment Grade Ratings from Standard & Poor's, Moody's, or Fitch Ratings
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	2
Wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties	$ 19.7
Percentage of wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties (in hundredths)	17.00%
No Investment Grade Ratings from External Credit Rating Agencies
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	6
Wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties	$ 58.1
Percentage of wholesale, trading and non-trading commodity and transmission activity credit exposure for the most significant counterparties (in hundredths)	50.00%