Stock Repurchases (Details) - USD ($) shares in Millions				1 Months Ended	12 Months Ended
	Oct. 19, 2016	Aug. 01, 2016	Feb. 10, 2016	Jul. 18, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2011
Authorized amount for share repurchase					$ 550,000,000			$ 2,000,000,000
Accelerated Share Repurchases, Initial Price Paid		$ 100,000,000	$ 200,000,000
Accelerated Share Repurchases, Number	0.2	0.5	1.3	0.2
Accelerated Share Repurchases, Percentages of Shares to be Repurchased		75.00%	75.00%
Repurchases of common stock					$ 300,000,000	$ 0	$ 550,300,000
Repurchased shares to satisfy minimum withholding obligations (in shares)					0.2	0.3
Repurchased shares to satisfy employee withholding tax obligations					$ 33,300,000	$ 32,000,000	$ 22,400,000