Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Income from continuing operations	$ 81.7	$ 103.6	$ 94.8	$ 39.5	$ 53.2	$ 86.3	$ 75.7	$ 36.5	$ 319.6	$ 251.7	$ 235.2
Less: dividends declared - common stock outstanding, restricted shares and restricted share units									(72.3)	(61.2)	(53.7)
Undistributed earnings									$ 247.3	$ 190.5	$ 181.5
Percent allocated to common shareholders									99.30%	99.50%	99.50%
Undistributed earnings allocated to common shareholders									$ 245.6	$ 189.5	$ 180.6
Add: dividends declared - common stock									71.4	60.4	53.4
Numerator for basic and diluted EPS									$ 317.0	$ 249.9	$ 234.0
Denominator (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding									64,844	64,170	63,471
Effect of dilutive securities:
Performance awards (in shares)									253	362	416
Stock options (in shares)									707	772	919
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion									65,804	65,304	64,806
Per share income from continuing operations:
Basic (in dollars per share)	$ 1.25	$ 1.59	$ 1.45	$ 0.60	$ 0.81	$ 1.34	$ 1.17	$ 0.57	$ 4.89	$ 3.89	$ 3.69
Diluted (in dollars per share)	$ 1.24	$ 1.56	$ 1.43	$ 0.59	$ 0.81	$ 1.31	$ 1.15	$ 0.56	$ 4.82	$ 3.83	$ 3.61
Basic weighted-average common shares outstanding									64,844	64,170	63,471
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units									65,304	64,512	63,797
Percent allocated to common shareholders									99.30%	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									$ 0.1	$ (0.4)	$ (25.4)
Net income attributable to common shareholders for basic and diluted earnings per share									$ 317.1	$ 249.5	$ 208.6
Antidilutive stock options excluded from EPS calculation (in shares)									257,500