Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Income from continuing operations	$ 115.3	$ 94.8	$ 183.8	$ 134.3
Less: dividends declared - common stock outstanding, restricted shares and restricted share units	(19.5)	(16.4)	(39.0)	(33.1)
Undistributed earnings	$ 95.8	$ 78.4	$ 144.8	$ 101.2
Percent allocated to common shareholders	99.30%	99.50%	99.30%	99.50%
Undistributed earnings allocated to common shareholders	$ 95.1	$ 78.0	$ 143.8	$ 100.7
Add: dividends declared - common stock	19.3	16.3	38.5	32.5
Numerator for basic and diluted EPS	$ 114.4	$ 94.3	$ 182.3	$ 133.2
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	64,246,000	65,010,000	64,131,000	64,943,000
Effect of dilutive securities:
Performance awards (in shares)	445,000	342,000	445,000	342,000
Stock options (in shares)	421,000	666,000	474,000	648,000
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	65,112,000	66,018,000	65,050,000	65,933,000
Per share income from continuing operations:
Basic (in dollars per share)	$ 1.78	$ 1.45	$ 2.84	$ 2.05
Diluted (in dollars per share)	$ 1.75	$ 1.43	$ 2.80	$ 2.02
Basic weighted-average common shares outstanding	64,246,000	65,010,000	64,131,000	64,943,000
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	64,703,000	65,344,000	64,587,000	65,276,000
Percent allocated to common shareholders	99.30%	99.50%	99.30%	99.50%
Income (loss) from discontinued operations and net income
Income (loss) from discontinued operations attributable to common shareholders for basic and diluted earnings per share	$ (0.1)	$ 0.1	$ (0.1)
Net income attributable to common shareholders for basic and diluted earnings per share	$ 114.3	$ 94.4	$ 182.2	$ 133.2
Antidilutive stock options excluded from EPS calculation (in shares)	54,000		387,000