Share Repurchase - Additional Information (Details) - USD ($) shares in Thousands, $ in Millions			1 Months Ended		9 Months Ended
	May. 18, 2015	Apr. 24, 2015	Feb. 28, 2015	Jan. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Jan. 28, 2015
Accelerated Share Repurchases [Line Items]
Payment for share repurchase					$ 263.1	$ 435.0
Share repurchase program, authorized amount							$ 500.0
Accelerated share repurchase agreements, contract value			$ 215.0
Evergreen Program
Accelerated Share Repurchases [Line Items]
Stock purchases, shares				500
Payment for share repurchase			$ 47.0	$ 41.2
2015 Repurchase Program
Accelerated Share Repurchases [Line Items]
Stock purchases, shares	83		2,600
Payment for share repurchase	$ 7.0		$ 168.0
Common stock issued		83
Accelerated share repurchase contract		$ 7.0