Equity (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Computation of basic and diluted earnings per share
Net income	$ 93.2	$ 55.3
Average common shares outstanding	44.3	44.5
Incremental shares from restricted stock units, stock options and convertible debentures	1.3	0.7
Average diluted shares outstanding	45.6	45.2
Basic earnings per share (in dollars per share)	$ 2.10	$ 1.24
Diluted earnings per share (in dollars per share)	$ 2.04	$ 1.22
Shares outstanding	44.1	44.9
Shares from options to purchase common stock, excluded from diluted earnings per share calculation	0	0.8
Number of shares of common stock issuable upon conversion of Series C debentures	1.8
Dilutive effect of the debentures (in shares)	0.4