Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Income from continuing operations	$ 102.3	$ 115.3	$ 163.8	$ 183.8
Less: dividends declared - common stock outstanding, restricted shares and restricted share units	(23.1)	(19.5)	(45.8)	(39.0)
Undistributed earnings	$ 79.2	$ 95.8	$ 118.0	$ 144.8
Percent allocated to common shareholders	99.40%	99.30%	99.40%	99.30%
Undistributed earnings available to common shareholders	$ 78.7	$ 95.1	$ 117.3	$ 143.8
Add: dividends declared - common stock	22.4	19.3	44.9	38.5
Income from continuing operations attributable to common shares	$ 101.1	$ 114.4	$ 162.2	$ 182.3
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	63,746	64,246	64,048	64,131
Effect of dilutive securities:
Performance awards (in shares)	69	445	69	445
Stock options (in shares)	325	421	356	474
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	64,140	65,112	64,473	65,050
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 1.59	$ 1.78	$ 2.53	$ 2.84
Diluted (in dollars per share)	$ 1.58	$ 1.75	$ 2.52	$ 2.80
Basic weighted-average common shares outstanding	63,746	64,246	64,048	64,131
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	64,160	64,703	64,462	64,587
Percent allocated to common shareholders	99.40%	99.30%	99.40%	99.30%
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	$ (0.1)	$ 0.3	$ (0.1)
Net income attributable to common shareholders for basic and diluted earnings per share	$ 101.1	$ 114.3	$ 162.5	$ 182.2
Antidilutive stock options excluded from EPS calculation (in shares)	379	54	285	387