Fair Value Measurements and Credit Concentration (Nonrecurring Fair Value Measurements) (Details) - Nonrecurring - USD ($) $ in Millions			1 Months Ended	6 Months Ended
		Mar. 31, 2014	Mar. 31, 2014	Jun. 30, 2014
Kerr Dam Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Loss			$ 18	$ 18	[1]
Carrying Amount | Kerr Dam Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Nonrecurring measurement asset value	[2]			47
Level 3 | Kerr Dam Project
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Nonrecurring measurement asset value		$ 29	29	$ 29
Fair value of net asset and (liability)	[3]	$ 29	$ 29
Discounted Cash Flow | Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Proprietary model used to calculate plant value	[3]	38.00%
Discounted Cash Flow | Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Proprietary model used to calculate plant value	[3]	38.00%
Discounted Cash Flow | Weighted Average | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Proprietary model used to calculate plant value	[3]	38.00%

[1] The loss on the Kerr Dam Project is included in "Other operation and maintenance" on the Statement of Income.

[2] Represents carrying value before fair value measurement.

[3] The range and weighted average represent the percentage of fair value derived from the unobservable inputs.