Fair Value Measurements - Losses Included in Earnings Relating to Assets Still Held at End of Period (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to assets still held at the end of period: $ (3) $ (12)
Derivative    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to assets still held at the end of period: (3) (13)
Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to assets still held at the end of period: $ 0 $ 1