Common Share Repurchases (details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Apr. 20, 2017
Common Share Repurchases disclosure
Number of shares repurchased (in shares)	2,600,000	8,300,000
Cost of shares repurchased	$ 328.0	$ 1,028.0
Average cost per share repurchased (in dollars per share)	$ 128.11	$ 124.12
Number of shares acquired to cover tax withholding costs and exercise costs (in shares)	964	500,000
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 0.1	$ 61.0
Additional share repurchase authorization			$ 5,000.0
Remaining capacity under share repurchase authorization	$ 4,910.0	$ 4,910.0