Stock Repurchases (Details) - USD ($) shares in Millions		3 Months Ended		12 Months Ended
	Feb. 08, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Repurchased During Period, Shares		0.5	1.0
Stock Repurchased During Period, Value		$ 100,000,000	$ 200,200,000
Authorized amount for share repurchase		2,500,000,000		$ 2,500,000,000	$ 500,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 446,000,000		$ 446,000,000
Accelerated Share Repurchases, Initial Price Paid	$ 150,000,000
Accelerated Share Repurchases, Percentages of Shares to be Repurchased	85.00%
Accelerated Share Repurchases, Number			0.7
Repurchased shares to satisfy minimum withholding obligations (in shares)				0.1	0.1
Repurchased shares to satisfy employee withholding tax obligations				$ 26,900,000	$ 26,100,000	$ 33,300,000