Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Income from continuing operations	$ 104.0	$ 139.3	$ 153.0	$ 77.4	$ 89.1	$ 96.9	$ 114.7	$ 57.9	$ 473.7	$ 358.6	$ 340.6
Less: dividends declared									(102.9)	(93.5)	(92.1)
Undistributed earnings									$ 370.8	$ 265.1	$ 248.5
Percent allocated to common shareholders									99.70%	99.70%	99.30%
Undistributed earnings allocated to common shareholders									$ 369.7	$ 264.3	$ 246.8
Add: dividends declared to common shares, restricted share units and vested and deferred restricted and performance shares									102.7	93.1	90.9
Income from continuing operations attributable to common shareholders									$ 472.4	$ 357.4	$ 337.7
Shares (in thousands):
Basic weighted-average shares outstanding (in thousands) (in shares)									56,913,000	60,393,000	63,073,000
Effect of dilutive securities:
Performance awards (in shares)									203,000	83,000	137,000
Stock options (in shares)									431,000	310,000	341,000
Diluted weighted-average shares outstanding (in shares)									57,547,000	60,786,000	63,551,000
Per share income from continuing operations attributable to common shares:
Basic (in dollars per share)	$ 1.84	$ 2.45	$ 2.68	$ 1.34	$ 1.50	$ 1.61	$ 1.88	$ 0.93	$ 8.30	$ 5.92	$ 5.36
Diluted (in dollars per share)	$ 1.81	$ 2.42	$ 2.65	$ 1.33	$ 1.49	$ 1.59	$ 1.87	$ 0.92	$ 8.21	$ 5.88	$ 5.32
Basic weighted-average shares outstanding (in thousands) (in shares)									56,913,000	60,393,000	63,073,000
Basic weighted-average common shares outstanding, unvested restricted shares expected to vest and restricted share units (in thousands) (in shares)									57,086,000	60,561,000	63,513,000
Income (loss) from discontinued operations and net income
Income (loss) from discontinued operations attributable to common shareholders for basic and diluted earnings per share									$ (0.9)	$ 251.8	$ 24.8
Net income attributable to common shareholders for basic and diluted earnings per share									$ 471.5	$ 609.2	$ 362.4
Antidilutive stock options excluded from EPS calculation (in shares)									1,000	813,000	321,000