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Capital Stock - Narrative (Details)
 3 Months Ended
Jan. 05, 2017
USD ($)
Mar. 31, 2017
USD ($)
director
common_share
$ / shares
Mar. 31, 2016
USD ($)
$ / shares
Nov. 30, 2016
USD ($)
May 31, 2014
USD ($)
Class of Stock [Line Items]          
Classes of common shares | common_share   2      
Stock repurchased during period, value   $ 1,760,000 $ 10,131,000    
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      
First Repurchase Plan [Member] | Common stock, Class A          
Class of Stock [Line Items]          
Stock repurchase program, authorized amount         $ 100,000,000
Stock repurchased during period, value $ 500,000   $ 10,100,000    
Stock repurchase program, remaining authorized amount   $ 0      
First Repurchase Plan [Member] | Common stock, Class A | Minimum          
Class of Stock [Line Items]          
Stock repurchased during period (in dollars per share) | $ / shares     $ 16.01    
First Repurchase Plan [Member] | Common stock, Class A | Maximum          
Class of Stock [Line Items]          
Stock repurchased during period (in dollars per share) | $ / shares     $ 19.51    
Second Repurchase Plan [Member] | Common stock, Class A          
Class of Stock [Line Items]          
Stock repurchase program, authorized amount       $ 100,000,000  
Stock repurchased during period, value   1,300,000      
Stock repurchase program, remaining authorized amount   $ 98,700,000      
Second Repurchase Plan [Member] | Common stock, Class A | Minimum          
Class of Stock [Line Items]          
Stock repurchased during period (in dollars per share) | $ / shares   $ 22.39      
Second Repurchase Plan [Member] | Common stock, Class A | Maximum          
Class of Stock [Line Items]          
Stock repurchased during period (in dollars per share) | $ / shares   $ 23.01