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Capital Stock and Share Based Compensation Plans - Summary of stock option transactions (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
$ / shares
shares
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding and Exercisable [Roll Forward]        
Options outstanding at beginning of period, number of shares | shares 486,914 996,879 1,703,876  
Options exercised in period, number of shares | shares (235,407) (509,965) (877,966)  
Options forfeited in period, number of shares | shares     170,969  
Options outstanding at end of period, number of shares | shares   486,914 996,879  
Options exercisable, number of shares | shares 251,507      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding and Exercisable, Weighted Average Exercise Price [Roll Forward]        
Options outstanding at beginning of period, weighted-average exercise price (USD per share) $ 6.81 $ 7.45 $ 8.92  
Options exercised in period, weighted-average exercise price (USD per share) 6.20 8.07 8.85  
Options forfeited in period, weighted-average exercise price (USD per share)     7.62  
Options outstanding at end of period, weighted-average exercise price (USD per share) 7.38 6.81 7.45  
Options Exercisable, Weighted Average Exercise Price (USD per share) $ 7.38 $ 6.81 $ 8.92
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding and Exercisable, Exercise Price Range [Roll Forward]        
Options outstanding at beginning of period, lower end of range of exercise price range (USD per share) 6 6 7  
Options outstanding at beginning of period, upper end of range of exercise price range (USD per share) 9 9 11  
Options exercised, lower end of exercise price range (USD per share) 6 6 8  
Options exercised, upper end of exercise price range (USD per share) 8 9 11  
Options forfeited, lower end of exercise price range (USD per share) 8  
Options forfeited, upper end of exercise price range (USD per share) 9  
Options impact of spin-off, lower end of exercise price range (USD per share)      
Options impact of spin-off, upper end of exercise price range (USD per share)      
Options outstanding at end of period, lower end of range of exercise price range (USD per share) 6 6 6  
Options outstanding at end of period, upper end of range of exercise price range (USD per share) 9 9 9  
Options exercisable, lower end of exercise price range (USD per share) 6 6 6  
Options exercisable, upper end of exercise price range 9 9 9  
Stock Options        
Options Exercisable, Aggregate Intrinsic Value | $ $ 2.1