Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator
Net income	$ 52.2	$ 25.8	$ 85.8	$ 33.1
Denominator
Basic shares outstanding - weighted-average shares outstanding	77.4	76.8	77.2	76.5
Effect of dilutive securities - dilutive stock-based compensation	0.1	0.1	0.1	0
Diluted shares outstanding - adjusted weighted-average shares after assumed conversion	77.5	76.9	77.3	76.5
Basic EPS	$ 0.67	$ 0.34	$ 1.11	$ 0.43
Diluted EPS	$ 0.67	$ 0.34	$ 1.11	$ 0.43
Antidilutive securities excluded from computation of earnings per share, amount			1.0	2.3
Earnings per share, range of exercise prices of Stock Options excluded from the calculation of diluted EPS, lower end of range	$ 19.54	$ 15.58	$ 19.54	$ 15.58
Earnings per share, range of exercise prices of Stock Options excluded from the calculation of diluted EPS, upper end of range	$ 40.83	$ 40.83	$ 40.83	$ 40.83