Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Income from continuing operations	$ 103.6	$ 86.3	$ 237.9	$ 198.4
Less: dividends declared - common stock outstanding, unvested restricted shares and restricted share units	(19.6)	(16.2)	(52.7)	(45.0)
Undistributed earnings	$ 84.0	$ 70.1	$ 185.2	$ 153.4
Percent allocated to common shareholders	99.50%	99.50%	99.50%	99.50%
Undistributed earnings allocated to common shareholders	$ 83.6	$ 69.7	$ 184.3	$ 152.6
Add: dividends declared - common stock	19.5	16.0	52.0	44.2
Numerator for basic and diluted EPS	$ 103.1	$ 85.7	$ 236.3	$ 196.8
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding	64,970	64,149	64,952	64,043
Effect of dilutive securities:
Performance awards (in shares)	348	306	348	306
Stock options (in shares)	669	992	752	966
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion	65,987	65,447	66,052	65,315
Per share income from continuing operations:
Basic (in dollars per share)	$ 1.59	$ 1.34	$ 3.64	$ 3.07
Diluted (in dollars per share)	$ 1.56	$ 1.31	$ 3.58	$ 3.01
Basic weighted-average common shares outstanding	64,970	64,149	64,952	64,043
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units	65,304	64,492	65,286	64,386
Percent allocated to common shareholders	99.50%	99.50%	99.50%	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		$ 1.0
Net income attributable to common shareholders for basic and diluted earnings per share	$ 103.1	$ 86.7	$ 236.3	$ 196.8