Employee Benefit Plans (Net Periodic Benefit Cost) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 31		$ 29		$ 100		$ 87
Interest cost	69		67		202		201
Expected return on plan assets	(116)		(108)		(345)		(323)
Amortization of prior service credit	0		0		2		2
Amortization of net loss	37		33		127		99
Settlements and curtailments	0	[1]	0	[1]	(2)	[1],[2]	0	[1],[2]
Net periodic benefit cost	21		21		84		66
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	10		11		34		33
Interest cost	19		20		56		60
Expected return on plan assets	(24)		(19)		(68)		(60)
Amortization of prior service credit	(3)		(4)		(9)		(10)
Amortization of net loss	2		2		6		5
Settlements and curtailments	0	[1]	(4)	[1]	(15)	[1],[2]	(4)	[1],[2]
Net periodic benefit cost	$ 4		$ 6		$ 4		$ 24

[1] 2012 amount relates primarily to the sale of Salem Harbor.

[2] 2013 amount relates primarily to the decommissioning of Kewaunee.