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Equity-Indexed Compensation Plans - Liability and Activity (Details) - LTIP - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Outstanding (in units)      
Outstanding at beginning of period (in units) 8.9 6.9 7.3
Granted (in units) 0.9 4.5 2.1
Vested (in units) (1.7) (1.9) (2.1)
Modified (in units) 0.0 0.0  
Cancelled or forfeited (in units) (0.8) (0.6) (0.4)
Outstanding at end of period (in units) 7.3 8.9 6.9
Weighted Average Grant Date Fair Value per Unit      
Outstanding at beginning of period (in dollars per unit) $ 29.62 $ 41.23 $ 41.45
Granted (in dollars per unit) 23.52 23.38 28.76
Vested (in dollars per unit) 42.12 45.91 28.91
Modified (in dollars per unit) (6.04) (8.21)  
Cancelled or forfeited (in dollars per unit) 26.99 37.19 44.56
Outstanding at end of period (in dollars per unit) $ 24.68 $ 29.62 $ 41.23
Units issued in connection with the settlement of vested awards, net of tax withholding (in units) 0.6 0.5 0.5
Units withheld for taxes (in units) 0.2 0.3 0.3
Vested awards settled in cash (in units) 0.9 1.1 1.3
Liability Awards      
Equity-Indexed Compensation Plans      
Accrued liability related to all outstanding LTIP awards and DERs $ 27 $ 38  
Short-term accrued liability related to all outstanding LTIP awards and DERs 15 25  
Long-term accrued liability related to all outstanding LTIP awards and DERs $ 12 $ 13