Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Income from continuing operations	$ 55.3	$ 60.0
Less: dividends declared - common stock outstanding, unvested restricted shares and restricted share units	(12.8)	(11.2)
Undistributed earnings	42.5	48.8
Percent allocated to common shareholders	99.50%	99.40%
Undistributed earnings allocated to common shareholders	42.3	48.5
Add: dividends declared - common stock	12.7	11.1
Numerator for basic and diluted EPS	55.0	59.6
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	63,253	61,913
Effect of dilutive securities:
Performance awards (in shares)	473	429
Stock options (in shares)	993	887
Denominator for diluted EPS: adjusted weighted average common shares outstanding and assumed conversion	64,719	63,229
Per share income from continuing operations:
Basic (in dollars per share)	$ 0.87	$ 0.96
Diluted (in dollars per share)	$ 0.85	$ 0.94
Basic weighted-average common shares outstanding	63,253	61,913
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	63,582	62,309
Percent allocated to common shareholders	99.50%	99.40%
Income from discontinued operations and net income
Loss from discontinued operations attributable to common shareholders for basic and diluted earnings per share	(0.1)
Net income attributable to common shareholders for basic and diluted earnings per share	$ 54.9	$ 59.6
Antidilutive stock options excluded from EPS calculation (in shares)		476