Fair Value Measurements (Schedule of Gains and Losses Included in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (18)	$ (15)
Operating Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	(1)
Electric Fuel and Other Energy-Related Purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (17)	$ (15)