Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 96.9	$ 79.1	$ 269.5	$ 231.7
Less: dividends declared on common stock outstanding, restricted shares and restricted share units	(24.1)	(23.2)	(69.7)	(69.0)
Undistributed earnings	$ 72.8	$ 55.9	$ 199.8	$ 162.7
Percent allocated to common shareholders	99.30%	99.30%	99.30%	99.40%
Undistributed earnings available to common shareholders	$ 72.3	$ 55.5	$ 198.4	$ 161.6
Add: dividends declared on common stock	23.8	23.2	69.0	68.1
Income from continuing operations attributable to common shares	$ 96.1	$ 78.7	$ 267.4	$ 229.7
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	59,826	62,432	60,712	63,503
Effect of dilutive securities:
Performance awards (in shares)	118	82	118	82
Stock options (in shares)	385	283	346	331
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	60,329	62,797	61,176	63,916
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 1.61	$ 1.27	$ 4.40	$ 3.62
Diluted (in dollars per share)	$ 1.59	$ 1.26	$ 4.37	$ 3.60
Basic weighted-average common shares outstanding	59,826	62,432	60,712	63,503
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	60,227	62,846	61,113	63,918
Income (loss) from discontinued operations and net income
Income from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ 2.8	$ 7.2	$ 251.9	$ 18.6
Net income attributable to common shareholders for basic and diluted earnings per share	$ 98.9	$ 85.9	$ 519.3	$ 248.4
Antidilutive stock options excluded from EPS calculation (in shares)	327	377	604	328