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Shareholders' Equity / Partners' Capital - Stock Options Activity (Details) - 2012 Equity Incentive Plan - Stock options - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Stock Options        
Outstanding at beginning of the period (in shares) 3,052,450 2,826,500 2,826,500  
Granted (in shares) 140,000 385,200    
Exercised (in shares) (7,500) (16,000)    
Forfeited (in shares) (10,000) (5,000)    
Outstanding at end of the period (in shares) 3,174,950 3,190,700 3,052,450 2,826,500
Exercisable at end of the period (in shares) 2,272,675 1,660,750    
Weighted Average Exercise Price        
Outstanding at beginning of the period (in dollars per share) $ 16.65 $ 15.69 $ 15.69  
Granted (in dollars per share) 19.40 23.38    
Exercised (in dollars per share) 16.62 16.07    
Forfeited (in dollars per share) 17.83 16.48    
Outstanding at end of the period (in dollars per share) 16.76 16.62 $ 16.65 $ 15.69
Exercisable at end of the period (in dollars per share) $ 16.16 $ 15.84    
Aggregate intrinsic value        
Weighted average remaining life (in years) 6 years 8 months 9 days 7 years 7 months 25 days 6 years 10 months 24 days 7 years 7 months 6 days
Weighted average exercisable (in years) 6 years 26 days 6 years 10 months 10 days    
Aggregate intrinsic value $ 11,748 $ 20,404 $ 16,421 $ 14,956
Aggregate intrinsic value exercised 22 117    
Aggregate intrinsic value exercisable $ 9,213 $ 11,824