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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Current tax (benefit)/expense:      
State income tax $ 2 $ 1 $ 2
Canadian federal and provincial income tax 36 (2) 13
Total current tax (benefit)/expense 38 (1) 15
Deferred tax (benefit)/expense:      
State income tax, deferred (2) 1  
Canadian federal and provincial income tax, deferred 9 (1) (9)
Total deferred tax (benefit)/expense 7   (9)
Total income tax (benefit)/expense $ 45 $ (1) $ 6