Earnings Per Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator:
Net income	$ 32.9	$ 21.1	$ 79.2	$ 61.3
Numerator for basic earnings per share - income available to common shareholders	32.9	21.1	79.2	61.3
Effect of dilutive securities:
After-tax interest expense on convertible securities				0.2
Numerator for diluted earnings per share - income available to common shareholders	$ 32.9	$ 21.1	$ 79.2	$ 61.5
Denominator:
Denominator for basic earnings per share - weighted average shares	46.5	46.2	46.4	46.1
Effect of dilutive securities
Equity compensation plans	0.6	0.4	0.6	0.5
Convertible preferred stock	0.1	0.1	0.1	0.1
Convertible securities				0.3
Denominator for diluted earnings per share - adjusted weighted average and assumed conversion	47.2	46.7	47.1	47.0
Basic earnings per share	$ 0.71	$ 0.45	$ 1.71	$ 1.33
Diluted earnings per share	$ 0.70	$ 0.45	$ 1.68	$ 1.31