Net Earnings (Loss) 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, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings (loss) available to common shareholders	$ 97.7	$ 111.6	$ 62.0	$ 10.3	$ (64.4)	$ 1.7	$ 271.3	$ (52.4)
Weighted-average common shares outstanding (in millions)
Weighted-average common shares outstanding - basic	54.3			58.2			54.7	58.4
Weighted-average common shares outstanding - diluted	55.2			58.5			55.4	58.4
Net earnings (loss) per share - basic	$ 1.80			$ 0.18			$ 4.96	$ (0.90)
Net earnings (loss) per share - diluted	$ 1.77			$ 0.18			$ 4.90	$ (0.90)
Effect of dilutive securities - stock options
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.2			0.0			0.1	0.0
Effect of other share-based awards
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.7			0.3			0.6	0.0