Share-Based Compensation Plans - Schedule of activity of stock options and awards of shares and stock units (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Beginning Balance	143,000	306,726	566,774
Weighted Average Price, Beginning Balance (in dollars per share)	$ 23.06	$ 23.42	$ 24.15
Shares, Granted	0	0	0
Weighted Average Price, Granted (in dollars per share)	$ 0.00	$ 0.00	$ 0.00
Shares, Expired	0	0	(30,000)
Weighted Average Price, Expired (in dollars per share)	$ 0.00	$ 0.00	$ 12.09
Shares, Exercised	(50,000)	(163,726)	(230,048)
Weighted Average Price, Exercised (in dollars per share)	$ 20.42	$ 23.73	$ 26.71
Shares, Ending Balance	93,000	143,000	306,726
Weighted Average Price, Ending Balance (in dollars per share)	$ 24.48	$ 23.06	$ 23.42
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares, Beginning Balance	492,288	605,488	634,298
Weighted Average Price, Beginning Balance (in dollars per share)	$ 44.93	$ 39.47	$ 34.16
Shares, Granted	198,179	191,936	241,274
Weighted Average Price, Granted (in dollars per share)	$ 68.33	$ 46.86	$ 45.23
Shares, Forfeited	(1,200)	(965)	(3,587)
Weighted Average Price, Forfeited (in dollars per share)	$ 60.68	$ 43.13	$ 29.56
Shares, Vested	(180,395)	(304,171)	(266,497)
Weighted Average Price, Vested (in dollars per share)	$ 44.57	$ 35.29	$ 32.17
Shares, Ending Balance	508,872	492,288	605,488
Weighted Average Price, Ending Balance (in dollars per share)	$ 54.13	$ 44.93	$ 39.47