Benefit Plans - Assets of Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
PEF [Member]
Assets Of Benefit Plans [Line Items]
Plan assets reclassified to investments	$ 39
Pension Benefits [Member]
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	122
Pension Benefits [Member] | PEC [Member]
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	60
Pension Benefits [Member] | PEF [Member]
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	$ 61