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Stock-Based Compensation - Restricted Stock Units (Details) - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Stock-based compensation      
Period for recognition   3 years  
LTIP Program | Restricted stock units      
Stock-based compensation      
Granted (in shares) 113,260 126,680  
Granted (in dollars per share) $ 42 $ 39  
Forfeited (in shares)   15,600 11,600