Common Share Repurchases (details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Apr. 20, 2017
Common Share Repurchases disclosure
Number of shares repurchased (in shares)	3,800,000	5,700,000
Cost of shares repurchased	$ 475.0	$ 700.0
Average cost per share repurchased	$ 123.04	$ 122.33
Number of shares acquired to cover tax withholding costs and exercise costs (in shares)	1,803	500,000
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 0.2	$ 61.0
Additional share repurchase authorization			$ 5,000.0
Remaining capacity under share repurchase authorization	$ 5,230.0	$ 5,230.0