Net (Loss) Earnings Per Share - Calculations of Net (Loss) Earnings Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net (loss) earnings available to common shareholders	$ (64.4)	$ 1.7	$ 127.3	$ 53.5	$ (62.7)	$ 180.8
Weighted-average common shares outstanding (in millions)
Weighted-average common shares outstanding - basic (in shares)	58.2		60.0		58.5	60.3
Weighted-average common shares outstanding - diluted (in shares)	58.2		60.4		58.5	60.7
Net (loss) earnings per share - basic (in dollars per share)	$ (1.11)		$ 2.12		$ (1.07)	$ 3.00
Net (loss) earnings per share - diluted (in dollars per share)	$ (1.11)		$ 2.11		$ (1.07)	$ 2.98
Effect of dilutive securities - stock options
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.0		0.1		0.0	0.1
Effect of other share-based awards
Weighted-average common shares outstanding (in millions)
Effect of dilutive securities	0.0		0.3		0.0	0.3