Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income	$ 57.9	$ 57.9
Less: dividends declared on common stock outstanding, restricted shares and restricted share units	(23.1)	(22.7)
Undistributed earnings	$ 34.8	$ 35.2
Percent allocated to common shareholders	99.30%	99.30%
Undistributed earnings available to common shareholders	$ 34.5	$ 34.9
Add: dividends declared on common stock	22.8	22.5
Income from continuing operations attributable to common shares	$ 57.3	$ 57.4
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	61,684	64,353
Effect of dilutive securities:
Performance awards (in shares)	131	107
Stock options (in shares)	349	388
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	62,164	64,848
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 0.93	$ 0.89
Diluted (in dollars per share)	$ 0.92	$ 0.88
Basic weighted-average common shares outstanding	61,684	64,353
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	62,117	64,822
Percent allocated to common shareholders	99.30%	99.30%
Income (loss) from discontinued operations and net income
Income from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ 250.0	$ 3.9
Net income attributable to common shareholders for basic and diluted earnings per share	$ 307.3	$ 61.3
Antidilutive stock options excluded from EPS calculation (in shares)	564	210