Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Income from continuing operations	$ 115.1	$ 86.4	$ 102.3	$ 61.5	$ 76.5	$ (9.5)	$ 115.3	$ 68.5	$ 365.3	$ 250.8	$ 319.6
Less: dividends declared - common stock outstanding, restricted shares and restricted share units									(92.1)	(84.5)	(72.3)
Undistributed earnings									273.2	166.3	247.3
Undistributed earnings allocated to common shareholders									271.3	165.0	245.6
Add: dividends declared - common stock									90.9	83.6	71.4
Income from continuing operations attributable to common shares									$ 362.2	$ 248.6	$ 317.0
Shares (in thousands):
Denominator for basic EPS: weighted-average common shares outstanding									63,073	64,226	64,844
Effect of dilutive securities:
Performance awards (in shares)									137	257	253
Stock options (in shares)									341	400	707
Denominator for diluted EPS: adjusted weighted-average common shares outstanding and assumed conversion									63,551	64,883	65,804
Per share income from continuing operations:
Basic (in dollars per share)	$ 1.84	$ 1.38	$ 1.59	$ 0.95	$ 1.18	$ (0.15)	$ 1.78	$ 1.06	$ 5.75	$ 3.87	$ 4.89
Diluted (in dollars per share)	$ 1.82	$ 1.37	$ 1.58	$ 0.94	$ 1.17	$ (0.15)	$ 1.75	$ 1.05	$ 5.71	$ 3.83	$ 4.82
Basic weighted-average common shares outstanding									63,073	64,226	64,844
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units									63,513	64,682	65,304
Percent allocated to common shareholders									99.30%	99.30%	99.30%
Income (loss) from discontinued operations and net income
Income (loss) from discontinued operations attributable to common shareholders for basic and diluted earnings per share									$ 0.2	$ (0.7)	$ 0.1
Net income attributable to common shareholders for basic and diluted earnings per share									$ 362.4	$ 248.0	$ 317.1
Antidilutive stock options excluded from EPS calculation (in shares)									320,600	23,100	257,500