Net Earnings Per Share - Calculation of Net Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings available to common shareholders	$ 158.3	$ 158.0	$ 143.4	$ 97.0	$ 216.3	$ 137.7	$ 117.0	$ 74.4	$ 556.7	$ 545.4	$ 443.7
Weighted-average common shares outstanding (in millions):
Weighted-average common shares outstanding - basic (in shares)									64.6	67.1	70.1
Weighted-average common shares outstanding - diluted (in shares)									65.1	67.9	70.8
Net earnings per share - basic (in dollars per share)	$ 2.56	$ 2.45	$ 2.18	$ 1.46	$ 3.26	$ 2.06	$ 1.74	$ 1.10	$ 8.62	$ 8.13	$ 6.33
Net earnings per share - diluted (in dollars per share)	$ 2.54	$ 2.43	$ 2.17	$ 1.45	$ 3.22	$ 2.04	$ 1.72	$ 1.09	$ 8.56	$ 8.04	$ 6.27
Stock options
Weighted-average common shares outstanding (in millions):
Effect of dilutive securities (in shares)									0.1	0.2	0.2
Other share-based awards
Weighted-average common shares outstanding (in millions):
Effect of dilutive securities (in shares)									0.4	0.6	0.5