Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Income from continuing operations	$ 113.0	$ 102.7	$ 256.2	$ 243.5
Less: dividends declared	(28.4)	(28.7)	(84.4)	(84.7)
Undistributed earnings	$ 84.6	$ 74.0	$ 171.8	$ 158.8
Percent allocated to common shareholders	99.70%	99.70%	99.70%	99.70%
Undistributed earnings available to common shareholders	$ 84.3	$ 73.8	$ 171.3	$ 158.3
Add: dividends declared to common shares, restricted share units and vested and deferred restricted and performance shares	28.2	28.4	84.1	84.3
Income from continuing operations attributable to common shareholders	$ 112.5	$ 102.2	$ 255.4	$ 242.6
Denominator (in thousands):
Basic weighted-average shares outstanding (in shares)	52,300,000	54,100,000	52,600,000	54,900,000
Effect of dilutive securities:
Performance awards (in shares)	200,000	200,000	100,000	200,000
Stock options (in shares)	500,000	200,000	500,000	300,000
Denominator for diluted EPS: weighted-average shares outstanding (in shares)	53,000,000.0	54,500,000	53,200,000	55,400,000
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 2.15	$ 1.89	$ 4.86	$ 4.42
Diluted (in dollars per share)	$ 2.12	$ 1.88	$ 4.80	$ 4.38
Basic weighted-average shares outstanding (in shares)	52,300,000	54,100,000	52,600,000	54,900,000
Basic weighted-average shares outstanding and unvested restricted shares expected to vest (in shares)	52,400,000	54,300,000	52,700,000	55,100,000
Percent allocated to common shareholders	99.70%	99.70%	99.70%	99.70%
Anti-dilutive stock options excluded from EPS calculation
Income (loss) from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ 29.0	$ (0.4)	$ 37.3	$ (4.0)
Net income attributable to common shareholders for basic and diluted earnings per share	$ 141.5	$ 101.8	$ 292.7	$ 238.6
Anti-dilutive stock options excluded from earnings per share calculation (in shares)	0	400,000	200,000	300,000