Share Transactions - Share repurchases (Details) - USD ($) shares in Millions, $ in Millions	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Treasury stock, shares acquired (shares)	0.1	6.9
Shares repurchased for treasury	$ 2.9	$ 184.5
Common shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program	386.3
Aggregate purchase price of shares acquired since inception of share repurchase program	$ 3,970.0
Remaining authorized repurchase amount	$ 160.9

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

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- Definition

Aggregate cumulative purchase price of common and preferred stock that have been repurchased since the inception of the share repurchase program.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount remaining of a stock repurchase plan authorized.

+ References

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- Definition

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

+ References

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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