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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Current tax (benefit)/expense:      
State income tax $ 2 $ 2 $ 1
Canadian federal and provincial income tax 51 36 (2)
Total current tax (benefit)/expense 53 38 (1)
Deferred tax expense:      
State income tax   (2) 1
Canadian federal and provincial income tax 1 9 (1)
Deferred income tax expense 1 7  
Total income tax (benefit)/expense $ 54 $ 45 $ (1)