Derivative Instruments (Details 4) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues		$ 1,164	$ 1,442	$ 2,779	$ 3,088
Natural gas contracts included in fuel and purchased energy expense		542	766	1,668	1,776
Interest expense		157	158	314	312
Gain (Loss) on Derivative Instruments, Net, Pretax		27	103	51	233
Gain (Loss) on Sale of Derivatives	[1],[2]	63	104	181	163
Mark-to-market gain (loss)	[3]	(36)	(1)	(130)	70
Power [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues	[1],[2]	(272)	115	(68)	234
Interest Rate Contract [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Interest expense	[3]	0	0	1	1
Mark-to-market gain (loss)	[3]	0	0	1	1
Energy Related Derivative [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	[1],[2]	63	104	181	163
Mark-to-market gain (loss)	[3]	(36)	(1)	(131)	69
Natural Gas [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Natural gas contracts included in fuel and purchased energy expense	[1],[2]	$ 299	$ (12)	$ 118	$ (2)

[1] Does not include the realized value associated with derivative instruments that settle through physical delivery.

[2] Includes amortization of acquisition date fair value of derivative activity related to the acquisition of Champion Energy.

[3] In addition to changes in market value on derivatives not designated as hedges, changes in mark-to-market gain (loss) also includes hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.