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Stock-Based Compensation - Restricted Stock Units (Details) - LTIP Program - Restricted stock units - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock-based compensation    
Period for recognition 3 years  
Granted (in shares) 113,260 124,780
Granted (in dollars per share) $ 42 $ 34
Forfeited (in shares) 0 0