Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Income from continuing operations	$ 102.3	$ 139.3	$ 239.5	$ 369.7
Less: dividends declared	(28.7)	(28.3)	(84.7)	(74.5)
Undistributed earnings	$ 73.6	$ 111.0	$ 154.8	$ 295.2
Percent allocated to common shareholders	99.70%	99.70%	99.70%	99.70%
Undistributed earnings available to common shareholders	$ 73.4	$ 110.7	$ 154.3	$ 294.3
Add: dividends declared on common stock, restricted share units and vested and deferred restricted and performance shares	28.4	28.2	84.3	74.3
Income from continuing operations attributable to common shares	$ 101.8	$ 138.9	$ 238.6	$ 368.6
Denominator (in thousands):
Basic weighted-average shares outstanding (in shares)	54,100,000	56,700,000	54,900,000	57,100,000
Effect of dilutive securities:
Performance awards (in shares)	200,000	200,000	200,000	200,000
Stock options (in shares)	200,000	400,000	300,000	300,000
Denominator for diluted EPS: weighted-average shares outstanding (in shares)	54,500,000	57,300,000	55,400,000	57,600,000
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 1.88	$ 2.45	$ 4.35	$ 6.46
Diluted (in dollars per share)	$ 1.87	$ 2.42	$ 4.31	$ 6.40
Basic weighted-average shares outstanding (in shares)	54,100,000	56,700,000	54,900,000	57,100,000
Basic weighted-average shares outstanding and unvested restricted shares expected to vest (in shares)	54,300,000	56,800,000	55,100,000	57,300,000
Percent allocated to common shareholders	99.70%	99.70%	99.70%	99.70%
Income (loss) from discontinued operations and net income
(Loss) income from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ 0.0	$ (1.4)	$ 0.0	$ 0.5
Net income attributable to common shareholders for basic and diluted earnings per share	$ 101.8	$ 137.5	$ 238.6	$ 369.1
Antidilutive stock options excluded from EPS calculation (in shares)	400,000	0	300,000	0