Benefit Plans - Assets of Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	$ 42
PEC
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	22
PEF
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	$ 20