Fair Value Measurements and Credit Concentration (Financial Instruments Not Recorded at Fair Value and Credit Concentration) (Details) $ in Millions	Sep. 30, 2015 USD ($) counterparty	Dec. 31, 2014 USD ($)
Credit Concentration Associated with Financial Instruments (Numeric) [Abstract]
Gross credit exposure from energy trading partners	$ 554
Net credit exposure to energy trading partners	351
Dollar exposure to top ten energy trading counterparties	$ 170
Percentage exposure to top ten energy trading counterparties	48.00%
Number of counterparties in top ten that have investment grade credit rating | counterparty	9
Percentage of top ten's exposure that consists of counterparties with investment grade credit rating	91.00%
Carrying Amount
Fair Value of Financial Instruments Not Recorded at Fair Value - Other
Long-term debt	$ 4,030	$ 2,218
Fair Value
Fair Value of Financial Instruments Not Recorded at Fair Value - Other
Long-term debt	$ 3,717	$ 2,204