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Stock-Based Compensation - Restricted Stock Units (Details) - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Stock-based compensation    
Period for recognition 3 years  
LTIP Program | Restricted stock units    
Stock-based compensation    
Granted (in shares) 124,450 113,260
Granted (in dollars per share) $ 39 $ 42
Forfeited (in shares) 0 0