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Stock-Based Compensation - Restricted Stock Units (Details) - LTIP Program - Restricted stock units - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Stock-based compensation      
Period for recognition   3 years  
Granted (in shares) 124,780 113,260  
Granted (in dollars per share) $ 34 $ 42  
Forfeited (in shares)   11,600 0