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Unit-Based Compensation (Phantom Common Unit Awards) (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2016
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Non-vested at beginning of the period, Weighted Average Grant Date Fair Value (in dollars per share) $ 31.89 $ 34.43
Granted, Weighted Average Grant Date Fair Value (in dollars per share) 28.86 28.31
Forfeited, Weighted Average Grant Date Fair Value (in dollars per share) 31.93 34.71
Non-vested at end of period, Weighted Average Grant Date Fair Value (in dollars per share) 30.96  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value $ 28.18 $ 45.48
Phantom common units [Member]    
Nonvested, Number of Shares [Roll Forward]    
Non-vested at beginning of period (in shares) 1,777,301 2,013,634
Granted (in shares) 420,300 203,867
Non-vested at end of period (in shares) 1,813,261  
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares 169,996 289,377
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares (214,344) (150,823)