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								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings available to common shareholders	$ 76.2	$ 10.3	$ (64.4)	$ 1.7	$ 138.8	$ 146.1	$ 127.3	$ 53.5	$ 23.8	$ 465.7	$ 556.7
Weighted-average common shares outstanding (in millions):
Weighted-average common shares outstanding - basic (in shares)									58.0	59.9	64.6
Weighted-average common shares outstanding - diluted (in shares)									58.3	60.3	65.1
Net earnings per share - basic	$ 1.34	$ 0.18	$ (1.11)	$ 0.03	$ 2.35	$ 2.44	$ 2.12	$ 0.88	$ 0.41	$ 7.78	$ 8.62
Net earnings per share - diluted	$ 1.33	$ 0.18	$ (1.11)	$ 0.03	$ 2.33	$ 2.42	$ 2.11	$ 0.88	$ 0.41	$ 7.72	$ 8.56
Effect of dilutive securities - stock options
Weighted-average common shares outstanding (in millions):
Effect of dilutive securities									0.0	0.0	0.1
Effect of other share-based awards
Weighted-average common shares outstanding (in millions):
Effect of dilutive securities									0.3	0.4	0.4