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Capital Stock and Share Based Compensation Plans - Narrative (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
director
common_share
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Nov. 30, 2016
USD ($)
Class of Stock [Line Items]        
Classes of common shares | common_share 2      
Minimum number of directors up for election to entitle shareholders to vote | director 3      
Minumum percent of directors up for election to entitle shareholders to vote 33.33%      
Stock repurchased during period, value $ 17,885,000 $ 44,401,000 [1] $ 16,222,000 [1]  
Number of shares available for future stock compensation grants | shares 3,900,000.0      
Total unrecognized compensation cost related to restricted stock, RSUs and performance shares $ 11,100,000      
Common stock, Class A        
Class of Stock [Line Items]        
Stock repurchase program, authorized amount       $ 100,000,000
Stock repurchased during period, value 17,900,000 $ 44,400,000 $ 16,200,000  
Outstanding amount authorized $ 83,000,000      
Common stock, Class A | Minimum        
Class of Stock [Line Items]        
Range of price of shares repurchased (in usd per share) | $ / shares $ 14.05 $ 12.84 $ 15.92  
Common stock, Class A | Maximum        
Class of Stock [Line Items]        
Range of price of shares repurchased (in usd per share) | $ / shares $ 23.01 $ 19.51 $ 24.96  
Stock Options | Common stock, Class A | Maximum        
Class of Stock [Line Items]        
Purchase price of stock options granted, percent of fair market value 100.00%      
Restricted Stock Units (RSUs)        
Class of Stock [Line Items]        
Weighted-average period of recognition, years 1 year 5 months      
Restricted Stock Units (RSUs) | Minimum        
Class of Stock [Line Items]        
Vesting period of stock options 3 years      
Restricted Stock Units (RSUs) | Maximum        
Class of Stock [Line Items]        
Vesting period of stock options 4 years      
[1] * Net of tax payments related to shares withheld for vested stock and RSUs of $4,576 in 2017, $2,681 in 2016 and $5,237 in 2015.