Benefit Plans - Contribution and Benefit Payment Expectations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined Contribution Benefit And Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, cost recognized	$ 12	$ 12	$ 35	$ 34
PEC [Member]
Defined Contribution Benefit And Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, cost recognized	7	6	18	18
PEF [Member]
Defined Contribution Benefit And Other Postretirement Plans Disclosure [Abstract]
Defined contribution plan, cost recognized	4	4	11	11
Pension Benefits [Member]
Contribution And Benefit Payment [Line Items]
2012 expected contributions	146		146
Pension Benefits [Member] | PEC [Member]
Contribution And Benefit Payment [Line Items]
2012 expected contributions	71		71
Pension Benefits [Member] | PEF [Member]
Contribution And Benefit Payment [Line Items]
2012 expected contributions	$ 74		$ 74