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Capital Stock and Share Based Compensation Plans - Summary of stock option transactions (Details) - Stock Options
12 Months Ended
Dec. 31, 2018
$ / shares
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 251,507
Options exercised in period, number of shares | shares (251,507)
Options outstanding at end of period, number of shares | shares 0
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) $ 7.38
Options exercised in period, weighted-average exercise price (USD per share) 7.38
Options outstanding at end of period, weighted-average exercise price (USD per share)
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
Options outstanding at beginning of period, upper end of range of exercise price range (USD per share) 9
Options exercised, lower end of exercise price range (USD per share) 6
Options exercised, upper end of exercise price range (USD per share) 9