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Stock-Based Plans - VDI Units (Details) - Performance-based VDI units - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
VDI units      
Vesting period 3 years    
Post vesting holding period restriction 3 years    
Unamortized compensation expense $ 1    
Performance-based liabilities paid during year $ 26    
Maximum      
VDI units      
Weighted average period of recognition of unamortized expense 1 year    
Corporate general and administrative expense      
VDI units      
Recognized compensation expense $ 8 $ 8 $ 11
Executives      
VDI units      
Granted (in shares) 249,204 296,052 430,970
2017 VDI Plan | Executives      
VDI units      
Expected amount of shares granted (in shares) 83,068    
Expected weighted average grant date fair value (in dollars per share) $ 53.35    
2016 VDI Plan | Executives      
VDI units      
Expected amount of shares granted (in shares)   92,094  
Expected weighted average grant date fair value (in dollars per share)   $ 51.62