Common Share Repurchases (Details) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Number of shares repurchased (in shares) | shares	2,600,000	5,500,000
Cost of shares repurchased	$ 375.0	$ 750.0
Average cost per share repurchased (in dollars per share) | $ / shares	$ 145.87	$ 137.15
Number of shares acquired to cover tax withholding costs and exercise costs (in shares) | shares	3,460	300,000
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 0.5	$ 47.0
Remaining capacity under share repurchase authorization	$ 2,540.0	$ 2,540.0