Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions			12 Months Ended			47 Months Ended	53 Months Ended
	Oct. 31, 2013	Aug. 01, 2011	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2017	Dec. 31, 2015	Mar. 31, 2014
Accelerated Share Repurchases, Settlement (Payment) or Receipt	$ 350.0
Accelerated Share Repurchases, Shares Repurchased	9.4
Accelerated Share Repurchases, Initial Price Paid Per Share	$ 29.69
Accelerated Share Repurchases, Final Shares Received								2.0
Accelerated Share Repurchases, Final Price Paid	$ 350.0
Stock Repurchased and Retired During Period, Value			$ (180.4)	$ (363.2)	$ (119.5)
Share Repurchase Plan [Member]
Stock Repurchase Program, Authorized Amount		$ 300.0		$ 750.0
Stock Repurchase Program, Period in Force		3 years				3 years 11 months
Stock Repurchase Program, Remaining Authorized Repurchase Amount					$ 42.9
Amended Share Repurchase Plan [Member]
Stock repurchase program, length of period						February 2014 through the end of 2017
Stock Repurchased and Retired During Period, Shares			4.5				28.9
Stock Repurchased and Retired During Period, Value			$ 180.4				$ 800.7
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 255.9				$ 255.9