Share-Based Compensation Plans - Schedule of activity of stock options and awards of shares and stock units (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Beginning Balance	1,310,697	1,796,377	3,400,541
Weighted Average Price, Beginning Balance (in dollars per share)	$ 18.12	$ 17.15	$ 15.30
Shares, Granted	0	0	11,702
Weighted Average Price, Granted (in dollars per share)	$ 0.00	$ 0.00	$ 27.39
Shares, Expired	0	0	(25,624)
Weighted Average Price, Expired (in dollars per share)	$ 0.00	$ 0.00	$ 28.13
Shares, Exercised	(743,923)	(485,680)	(1,590,242)
Weighted Average Price, Exercised (in dollars per share)	$ 13.52	$ 14.55	$ 13.09
Shares, Ending Balance	566,774	1,310,697	1,796,377
Weighted Average Price, Ending Balance (in dollars per share)	$ 24.15	$ 18.12	$ 17.15
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	610,803	588,907	486,850
Weighted Average Price, Beginning Balance (in dollars per share)	$ 31.17	$ 28.56	$ 27.75
Shares, Granted	176,418	192,617	340,518
Weighted Average Price, Granted (in dollars per share)	$ 43.06	$ 36.26	$ 27.90
Shares, Forfeited	(500)	(15,298)	0
Weighted Average Price, Forfeited (in dollars per share)	$ 43.76	$ 29.38	$ 0.00
Shares, Vested	(152,423)	(155,423)	(238,461)
Weighted Average Price, Vested (in dollars per share)	$ 32.46	$ 27.77	$ 25.96
Shares, Ending Balance	634,298	610,803	588,907
Weighted Average Price, Ending Balance (in dollars per share)	$ 34.16	$ 31.17	$ 28.56