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Income Taxes (Details 2) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income Taxes      
Income before tax $ 1,181 $ 1,039 $ 513
Partnership earnings not subject to current Canadian tax (1,046) (909) (509)
Partnership earnings subject to current Canadian tax 135 130 4
Canadian federal and provincial corporate tax rate (as a percent) 25.00% 27.00% 28.00%
Income tax at statutory rate 34 35 1
Current tax (benefit)/expense:      
Canadian withholding tax 18 12  
Canadian period tax adjustments resulting from tax rate changes and tax return filings (3) (9)  
Canadian permanent differences between book and tax 2 (2) (3)
State income tax 2 2 1
Total current tax (benefit)/expense 53 38 (1)
Deferred tax expense:      
State deferred income tax (benefit)/expense   (2) 1
Canadian deferred tax (benefit)/expense as a result of book versus tax differences 1 9 (1)
Deferred income tax expense 1 7  
Total income tax (benefit)/expense 54 45 (1)
Deferred tax assets:      
Book accruals in excess of current tax deductions 28 18  
Total deferred tax assets 28 18  
Deferred tax liabilities:      
Property and equipment in excess of tax values (397) (147)  
Total deferred tax liabilities (397) (147)  
Net deferred tax liabilities (369) (129)  
Increase in net deferred taxes liabilities $ 236