Stock Incentive Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grant	8,000,000
Status of Stock Options
Shares outstanding under option, beginning balance	1,997,000	2,872,000	4,440,000
Shares exercised	(283,000)	(870,000)	(1,568,000)
Shares forfeited		(5,000)
Shares outstanding under option, ending balance	1,714,000	1,997,000	2,872,000	4,440,000
Weighted average exercise price, beginning balance	$ 15.82	$ 15.33	$ 14.68
Weighted average exercise price, exercised	$ 14.04	$ 14.17	$ 13.43
Weighted average exercise price, ending balance	$ 16.11	$ 15.82	$ 15.33	$ 14.68
Weighted average exercise price, forfeited		$ 17.48
Weighted average remaining contractual life outstanding, beginning balance		2 years 7 months	3 years 5 months	4 years 1 month
Aggregate intrinsic value, outstanding, beginning balance	$ 80.0	$ 58.8	$ 76.3
Aggregate intrinsic value, exercised	13.8	25.5	40.1
Aggregate intrinsic value, outstanding, ending balance	66.5	80.0	58.8	76.3
Weighted average remaining contractual life, outstanding	1 year 8 months
Share under option, option exercisable	1,714,000
Weighted average exercise price, options exercisable	$ 16.11
Weighted average remaining contractual life option exercisable	1 year 8 months
Aggregate intrinsic value, option exercisable	66.5
Proceeds from stock based compensation - gross	4.0	12.3	20.9
Tax benefit (loss)	12.9	20.7	31.1
Proceeds from stock options exercised, including the tax benefit	$ 16.9	$ 33.0	$ 52.0