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Share Transactions - Share repurchases (Details) - Common shares - USD ($) shares in Millions, $ in Millions | 9 Months Ended
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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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- Definition Amount remaining of a stock repurchase plan authorized. + References No definition available. + Details
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