Derivative Instruments (Details 3) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives									$ 110	[1]	$ 86	[1]	$ 230	[1]
Unrealized Gain (Loss) on Derivatives									353	[2]	(12)	[2]	72	[2]
Derivatives contracts included in operating revenues	1,939	2,187	1,939	1,965	1,438	2,050	1,572	1,241	8,030		6,301		5,478
Derivatives contracts included in fuel and purchased energy									4,892		3,736		3,024
Interest expense									645		696		736
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments									0		0		(14)
Gain (Loss) on Derivative Instruments, Net, Pretax									463		74		302
Energy Related Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives									110	[1]	86	[1]	387	[1]
Unrealized Gain (Loss) on Derivatives									342	[2]	(14)	[2]	(82)	[2]
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives									0	[1]	0	[1]	(157)	[1]
Unrealized Gain (Loss) on Derivatives									11	[2]	2	[2]	154	[2]
Interest expense									11		2		11
Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives contracts included in operating revenues									384		(119)		187
Natural Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives contracts included in fuel and purchased energy									$ 68		$ 191		$ 118

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

[2] In addition to changes in market value on derivatives not designated as hedges, changes in mark-to-market gain (loss) also includes de-designation of interest rate swap cash flow hedges and related reclassification from AOCI into earnings, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.