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Acquisitions (Acquisition of Hardin Street Marine LLC) (Details)
$ in Millions
12 Months Ended
Feb. 01, 2018
Mar. 01, 2017
USD ($)
Mar. 31, 2016
USD ($)
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Business Acquisition [Line Items]            
Aggregate percentage of general partner interest 2.00% 2.00% 2.00% 2.00%    
Contribution from MPC       $ 1,046 $ 675 $ 225
Number of tow boats     18      
Number of barges     219      
MPC            
Business Acquisition [Line Items]            
Contribution from MPC   $ 6 $ 15      
Percent of volumes shipped     60.00%      
MPC | General Partner            
Business Acquisition [Line Items]            
Contribution from MPC       $ 0 $ 0 $ 141
Hardin Street Marine            
Business Acquisition [Line Items]            
Equity interest issued or issuable, value assigned     $ 600      
Effective date of acquisition     Mar. 31, 2016      
Hardin Street Marine | Limited Partner            
Business Acquisition [Line Items]            
Equity interest issued or issuable, number of shares | shares     22,534,002      
Equity interest issued or issuable, fair value assigned     $ 669      
Hardin Street Marine | General Partner            
Business Acquisition [Line Items]            
Equity interest issued or issuable, number of shares | shares     459,878      
Equity interest issued or issuable, fair value assigned     $ 14