Common Share Repurchases (Details) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Number of shares repurchased (in shares) | shares	2,700,000	5,200,000
Cost of shares repurchased	$ 350.0	$ 700.0
Average cost per share repurchased (in dollars per share) | $ / shares	$ 129.66	$ 135.47
Number of shares acquired to cover tax withholding costs and exercise costs (in shares) | shares	1,374.000000	300,000
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 0.2	$ 51.0
Remaining capacity under share repurchase authorization	$ 3,860.0	$ 3,860.0