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 activities with credit exposure	10
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 activities with credit exposure	4
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	$ 48.8
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	16.00%
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 activities with credit exposure	6
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	$ 75.0
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties	25.00%