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Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]    
Emission allowances $ 9 $ 10
Renewable energy credits 40 39
Contract intangible assets 64 65
Intangible assets, gross 113 114
Less: accumulated amortization 28 28
Intangible assets, net 85 86
Less current intangible assets 15 13
Intangible assets noncurrent $ 70 $ 73