Share Transactions - Share repurchases (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) shares
Class of Stock [Line Items]
Treasury stock, shares acquired (shares) | shares	6,522,645
Shares repurchased for treasury	$ 173.6
Common shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program | shares	382,200,000
Aggregate purchase price of shares acquired since inception of share repurchase program	$ 3,860.0
Remaining authorized repurchase amount	$ 272.9