Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Numerator:
Net income	$ 31.6	$ 32.9	$ 26.4	$ 19.9	$ 19.5	$ 21.1	$ 21.5	$ 18.7	$ 110.8	$ 80.8	$ 81.4
Numerator for basic earnings per share - income available to common shareholders									110.8	80.8	81.4
Effect of dilutive securities:
After-tax interest expense on convertible securities										0.2	1.3
Numerator for diluted earnings per share - income available to common shareholders									$ 110.8	$ 81.0	$ 82.7
Denominator:
Denominator for basic earnings per share - weighted average shares									46.4	46.1	46.6
Effect of dilutive securities:
Equity compensation plans									0.7	0.5	0.4
Convertible preferred stock									0.1	0.1	0.1
Convertible securities										0.3	1.7
Denominator for diluted earnings per share - adjusted weighted average and assumed conversion									47.2	47.0	48.8
Basic earnings per share	$ 0.68	$ 0.71	$ 0.57	$ 0.43	$ 0.42	$ 0.45	$ 0.47	$ 0.41	$ 2.39	$ 1.75	$ 1.74
Diluted earnings per share	$ 0.67	$ 0.70	$ 0.56	$ 0.42	$ 0.42	$ 0.45	$ 0.46	$ 0.40	$ 2.35	$ 1.72	$ 1.70