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Share Transactions - Share repurchases (Details) - Common shares - USD ($) shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program	412.0
Aggregate purchase price of shares acquired since inception of share repurchase program	$ 4,920.0
Treasury stock, shares acquired (shares)	22.8	2.6
Shares repurchased for treasury	$ 872.2	$ 75.5
Remaining authorized repurchase amount	$ 44.3

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Cumulative number of shares that have been repurchased since the inception of the share repurchase program.

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Number of shares that have been repurchased during the period and are being held in treasury.

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