Net Earnings Per Share - Calculations of Net Earnings Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings available to common shareholders	$ 146.1	$ 158.0	$ 326.9	$ 398.4
Weighted-average common shares outstanding (in millions)
Weighted-average common shares outstanding - basic (in shares)	59.9	64.5	60.2	65.6
Weighted-average common shares outstanding - diluted (in shares)	60.3	65.0	60.6	66.1
Net earnings per share - basic (in dollars per share)	$ 2.44	$ 2.45	$ 5.43	$ 6.08
Net earnings per share - diluted (in dollars per share)	$ 2.42	$ 2.43	$ 5.40	$ 6.03
Effect of dilutive securities - stock options
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.1	0.1	0.1	0.1
Effect of other share-based awards
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.3	0.4	0.3	0.4