Stock Repurchases (Details) - USD ($) shares in Millions				12 Months Ended
	Jul. 27, 2017	Apr. 28, 2017	Feb. 09, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Authorized amount for share repurchase				$ 2,000,000,000	$ 550,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 396,000,000
Accelerated Share Repurchases, Initial Price Paid	$ 75,000,000	$ 100,000,000	$ 75,000,000
Accelerated Share Repurchases, Percentages of Shares to be Repurchased	85.00%	85.00%	85.00%
Accelerated Share Repurchases, Number	0.4	0.6	0.5
Repurchased shares to satisfy minimum withholding obligations (in shares)				0.1	0.2
Repurchased shares to satisfy employee withholding tax obligations				$ 26,100,000	$ 33,300,000	$ 32,000,000