Earnings Per Share - Computation of Basic and Diluted Net Income Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income	$ 53.8	$ 32.9	$ 107.6	$ 79.2
Denominator:
Weighted average shares outstanding - basic	46.9	46.5	46.8	46.4
Effect of dilutive securities:
Equity compensation plans	0.6	0.6	0.6	0.6
Convertible preferred stock	0.1	0.1	0.1	0.1
Weighted average shares outstanding - diluted	47.6	47.2	47.5	47.1
Basic earnings per share	$ 1.15	$ 0.71	$ 2.30	$ 1.71
Diluted earnings per share	$ 1.13	$ 0.70	$ 2.26	$ 1.68