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Repurchase of Common Stock (Details) - Stock Repurchase Plan - USD ($)
6 Months Ended 24 Months Ended
Aug. 17, 2016
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2015
Class of Stock [Line Items]        
Stock repurchase plan, amount approved       $ 50,000,000
Treasury stock repurchased (in shares)   0 146,000  
Treasury stock acquired, average price (in dollars per share)     $ 20.48  
Treasury stock acquired, aggregate cost     $ 3,000,000  
Stock repurchase plan, remaining amount approved   $ 25,100,000    
Chairman and CEO        
Class of Stock [Line Items]        
Stock repurchase plan, amount approved (in shares) 1,000,000      
Stock repurchase plan, amount approved each month from CEO $ 2,000,000      
Average daily price of common stock for the preceding month discount, percent 2.00%      
Treasury stock repurchased (in shares)     1,000,000  
Treasury stock acquired, average price (in dollars per share)     $ 21.92  
Treasury stock acquired, aggregate cost     $ 21,900,000