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Income Taxes - Reconciliation and Deferred Tax Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Examination [Line Items]      
Income before tax $ 2,414 $ 902 $ 755
Partnership earnings not subject to current Canadian tax (1,690) (756) (723)
Partnership earnings subject to current Canadian tax $ 724 $ 146 $ 32
Canadian federal and provincial corporate tax rate 27.00% 27.00% 27.00%
Income tax at statutory rate $ 195 $ 39 $ 8
Canadian withholding tax 0 2 13
Canadian permanent differences and rate changes 0 2 2
State income tax 3 1 2
Total income tax expense 198 44 $ 25
Deferred tax assets:      
Derivative instruments 0 74  
Book accruals in excess of current tax deductions 21 22  
Net operating losses 2 3  
Total deferred tax assets 23 99  
Deferred tax liabilities:      
Property and equipment in excess of tax values (449) (455)  
Derivative instruments (31)    
Other (42) (50)  
Total deferred tax liabilities (522) (505)  
Net deferred tax liabilities (499) (406)  
Other long-term assets, net      
Deferred tax liabilities:      
Net deferred tax assets 2 3  
Other long-term liabilities and deferred credits      
Deferred tax liabilities:      
Net deferred tax liabilities $ (501) $ (409)