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Income Taxes - Reconciliation and Deferred Tax Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Examination [Line Items]      
Income before tax $ 902 $ 755 $ 1,006
Partnership earnings not subject to current Canadian tax (756) (723) (773)
Partnership earnings subject to current Canadian tax $ 146 $ 32 $ 233
Canadian federal and provincial corporate tax rate 27.00% 27.00% 26.00%
Income tax at statutory rate $ 39 $ 8 $ 61
Canadian withholding tax 2 13 14
Canadian permanent differences and rate changes 2 2 24
State income tax 1 2 1
Total income tax expense 44 25 $ 100
Deferred tax assets:      
Derivative instruments 74 49  
Book accruals in excess of current tax deductions 22 24  
Net operating losses 3 4  
Total deferred tax assets 99 77  
Deferred tax liabilities:      
Property and equipment in excess of tax values (455) (394)  
Other (50) (41)  
Total deferred tax liabilities (505) (435)  
Net deferred tax liabilities (406) (358)  
Other long-term assets, net      
Deferred tax liabilities:      
Net deferred tax assets 3 4  
Other long-term liabilities and deferred credits      
Deferred tax liabilities:      
Net deferred tax liabilities $ (409) $ (362)