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Capital Stock (Details)
 6 Months Ended
Jun. 30, 2018
USD ($)
director
common_share
$ / shares
Jun. 30, 2017
USD ($)
$ / shares
Nov. 30, 2016
USD ($)
Class of Stock [Line Items]      
Classes of common shares | common_share 2    
Stock repurchased during period, value $ 4,409,000 $ 6,257,000  
Minimum      
Class of Stock [Line Items]      
Minimum number of directors up for election to entitle shareholders to vote | director 0.3333    
Maximum      
Class of Stock [Line Items]      
Minimum number of directors up for election to entitle shareholders to vote | director 3    
Common stock, Class A      
Class of Stock [Line Items]      
Stock repurchase program, authorized amount     $ 100,000,000
Stock repurchased during period, value $ 4,400,000 $ 6,300,000  
Stock repurchase program, remaining authorized amount $ 78,200,000    
Common stock, Class A | Minimum      
Class of Stock [Line Items]      
Stock repurchased during period (in dollars per share) | $ / shares $ 13.29 $ 16.82  
Common stock, Class A | Maximum      
Class of Stock [Line Items]      
Stock repurchased during period (in dollars per share) | $ / shares $ 16.86 $ 23.01