Share Capital (Movements in Treasury Shares) (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Movement in Treasury Shares [Roll Forward]
Beginning balance	128.2	103.0	92.4
Acquisition of common shares (shares)	3.4	34.7	16.0
Distribution of common shares	(9.3)	(9.2)	(5.2)
Common shares distributed to meet ESPP obligation	(0.7)	(0.3)	(0.2)
Ending balance	121.6	128.2	103.0