Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 114.7	$ 94.7	$ 172.6	$ 152.6
Less: dividends declared on common stock outstanding, restricted shares and restricted share units	(22.5)	(23.1)	(45.6)	(45.8)
Undistributed earnings	$ 92.2	$ 71.6	$ 127.0	$ 106.8
Percent allocated to common shareholders	99.30%	99.40%	99.40%	99.40%
Undistributed earnings available to common shareholders	$ 91.6	$ 71.2	$ 126.2	$ 106.2
Add: dividends declared on common stock	22.4	22.4	45.2	44.9
Income from continuing operations attributable to common shares	$ 114.0	$ 93.6	$ 171.4	$ 151.1
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	60,641	63,746	61,159	64,048
Effect of dilutive securities:
Performance awards (in shares)	103	69	103	69
Stock options (in shares)	315	325	331	356
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	61,059	64,140	61,593	64,473
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 1.88	$ 1.47	$ 2.80	$ 2.35
Diluted (in dollars per share)	$ 1.87	$ 1.46	$ 2.78	$ 2.34
Basic weighted-average common shares outstanding	60,641	63,746	61,159	64,048
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	61,038	64,160	61,556	64,462
Percent allocated to common shareholders	99.30%	99.40%	99.40%	99.40%
Income (loss) from discontinued operations and net income
(Loss) income from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ (0.9)	$ 7.5	$ 249.1	$ 11.4
Net income attributable to common shareholders for basic and diluted earnings per share	$ 113.0	$ 101.1	$ 420.4	$ 162.5
Antidilutive stock options excluded from EPS calculation (in shares)	711	379	644	285