Common Share Repurchases (Details) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Number of shares repurchased (in shares) | shares	2,500,000	8,000,000.0
Cost of shares repurchased	$ 375.0	$ 1,130.0
Average cost per share repurchased (in dollars per share) | $ / shares	$ 147.23	$ 140.35
Number of shares acquired to cover tax withholding costs and exercise costs (in shares) | shares	628	300,000
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 0.1	$ 47.0
Remaining capacity under share repurchase authorization	$ 2,160.0	$ 2,160.0