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Capital Stock (Details 2) - Class A - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Oct. 31, 2011
Shares purchased outside of stock repurchase program      
Total cost of shares purchased outside of publicly announced stock repurchase program $ 6.7    
Performance shares      
Shares purchased outside of stock repurchase program      
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares) 55,935    
Equity compensation plan      
Shares purchased outside of stock repurchase program      
Total cost of shares purchased outside of publicly announced stock repurchase program $ 0.4    
Average price per share for shares purchased (in dollars per share) $ 111.55    
Equity compensation plan | Performance shares      
Shares purchased outside of stock repurchase program      
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares) 3,785    
Stock compensation plan for outside directors      
Shares purchased outside of stock repurchase program      
Total cost of shares purchased outside of publicly announced stock repurchase program $ 0.6    
Average price per share for shares purchased (in dollars per share) $ 118.56    
Stock compensation plan for outside directors | Performance shares      
Shares purchased outside of stock repurchase program      
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares) 5,266    
Long-term incentive plan      
Shares purchased outside of stock repurchase program      
Total cost of shares purchased outside of publicly announced stock repurchase program $ 5.7    
Average price per share for shares purchased (in dollars per share) $ 122.40    
Long-term incentive plan | Performance shares      
Shares purchased outside of stock repurchase program      
Shares purchased outside of publicly announced stock repurchase program for share-based compensation awards (in shares) 46,884    
Stock repurchase program      
Stock repurchases      
Amount of authorized stock repurchases approved for continuation under the current program     $ 150.0
Shares repurchased under stock repurchase program (in shares) 0 0  
Approximate amount of repurchase authority remaining under the current stock repurchase program $ 17.8