Stock Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Stock-Based Compensation arrangements
Stock based compensation expense	$ 51.8	$ 33.5	$ 39.5
Tax benefits realized for stock-based compensation	18.9
Restricted Stock Awards
Stock-Based Compensation arrangements
Stock based compensation expense	0.2	1.1	1.2
Summary of restricted stock award activity
Outstanding at beginning of year (in shares)	71,508	476,388	528,652
Granted (in shares)	7,200	13,100	39,200
Vested (in shares)	(50,902)	(402,800)	(29,464)
Forfeited (in shares)		(15,180)	(62,000)
Outstanding at the end of the period (in shares)	27,806	71,508	476,388
Weighted-Average Grant Date Fair Value per Share
Outstanding at the beginning of the period (in dollars per share)	$ 11.82	$ 9.18	$ 8.89
Granted (in dollars per share)	$ 35.70	$ 18.42	$ 13.83
Vested (in dollars per share)	$ 10.44	$ 9.03	$ 8.41
Forfeited (in dollars per share)		$ 8.80	$ 8.80
Outstanding at the end of the period (in dollars per share)	$ 20.53	$ 11.82	$ 9.18
Additional disclosures
Aggregate intrinsic value	1.1
Weighted-average remaining contractual term of non-vested shares	1 year
Unamortized compensation expense	0.4
Total fair value of shares vested	$ 1.6	$ 3.6	$ 0.2
Restricted Stock Awards | Greater than
Stock-Based Compensation arrangements
Annual forfeiture rate assumption (as a percent)	6.00%
Restricted Stock Awards | Less than or equal to
Stock-Based Compensation arrangements
Annual forfeiture rate assumption (as a percent)	7.00%
Restricted Stock - 3 Year Service Awards
Stock-Based Compensation arrangements
Vesting period	3 years
Restricted Stock - Graded Vesting | Greater than
Stock-Based Compensation arrangements
Vesting period	3 years
Restricted Stock - Graded Vesting | Less than or equal to
Stock-Based Compensation arrangements
Vesting period	4 years