Decommissioning Trust Funds (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Securities held
Fair Value	$ 3,788.0	$ 3,596.0
Total Unrealized Gains	538	503
Total Unrealized Losses	19	21
Equity securities [Member]
Securities held
Fair Value	2,129.0	2,076.0
Total Unrealized Gains	423	436
Total Unrealized Losses	14	9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	130	15
More than 12 months Fair Value	43	105
Total Fair Value	173	120
Less than 12 Months Gross Unrealized Losses	9	1
More than 12 Months Gross Unrealized Losses	5	8
Total Gross Unrealized Losses	14	9
Debt Securities [Member]
Securities held
Fair Value	1,659.0	1,520.0
Total Unrealized Gains	115	67
Total Unrealized Losses	5	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	123	474
More than 12 months Fair Value	60	4
Total Fair Value	183	478
Less than 12 Months Gross Unrealized Losses	3	11
More than 12 Months Gross Unrealized Losses	2	1
Total Gross Unrealized Losses	$ 5	$ 12