Employee Benefit Plans (Components of AOCI and Regulatory Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits [Member]
Net actuarial loss	$ 96	$ 59	$ 38
Prior service (credit) cost	3	3	4
Other Postretirement Benefits [Member]
Net actuarial loss	12	12	30
Prior service (credit) cost	(13)	(7)	(7)
2012 [Member] | Pension Benefits [Member]
Net actuarial loss	132
Prior service (credit) cost	3
2012 [Member] | Other Postretirement Benefits [Member]
Net actuarial loss	6
Prior service (credit) cost	$ (13)