Capital Stock and Share Based Compensation Plans - Summary of stock option transactions (Details)	12 Months Ended
	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / shares shares	Dec. 31, 2014 $ / shares shares	Dec. 31, 2013 $ / 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	996,879	1,703,876	3,370,048
Options exercised in period, number of shares | shares	(509,965)	(877,966)	(1,662,055)
Options forfeited in period, number of shares | shares			(4,117)
Options impact of spin-off, number of shares | shares		170,969
Options outstanding at end of period, number of shares | shares		996,879	1,703,876
Options exercisable, number of shares | shares	486,914
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)	$ 7.45	$ 8.92	$ 9.46
Options exercised in period, weighted-average exercise price (USD per share)	8.07	8.85	10.01
Options forfeited in period, weighted-average exercise price (USD per share)			10.45
Options impact of spin-off, weighted-average exercise price (USD per share)		7.62
Options outstanding at end of period, weighted-average exercise price (USD per share)	6.81	7.45	8.92
Options Exercisable, Weighted Average Exercise Price (USD per share)	$ 6.81	$ 7.45	$ 8.92	$ 9.46
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	7	7
Options outstanding at beginning of period, upper end of range of exercise price range (USD per share)	9	11	11
Options exercised, lower end of exercise price range (USD per share)	8	8	9
Options exercised, upper end of exercise price range (USD per share)	9	11	11
Options forfeited, lower end of exercise price range (USD per share)			10
Options forfeited, upper end of exercise price range (USD per share)			11
Options impact of spin-off, lower end of exercise price range (USD per share)		$ 6
Options impact of spin-off, upper end of exercise price range (USD per share)		$ 9
Options outstanding at end of period, lower end of range of exercise price range (USD per share)	6	6	7
Options outstanding at end of period, upper end of range of exercise price range (USD per share)	9	9	11
Options exercisable, lower end of exercise price range (USD per share)	6	6	7
Options exercisable, upper end of exercise price range	9	9	11