Employee Benefit Plans (Components of AOCI and Regulatory Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 132	$ 96	$ 59
Prior service (credit) cost	3	3	3
Pension Benefits | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	185
Prior service (credit) cost	3
Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	6	12	12
Prior service (credit) cost	(13)	(13)	(7)
Other Postretirement Plans | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	9
Prior service (credit) cost	$ (12)