Common Stock and Stock-Based Compensation Stockholders Equity (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Nov. 16, 2018	Dec. 31, 2017
Share Repurchases [Line Items]
Shares_repurchased_settled_in_subsequent_month					$ 4.0
Stock Repurchase Program, Authorized Amount		$ 350.0						$ 500.0
Stock Repurchase Program, Number of Shares Authorized to be Repurchased		13,700,000
Accelerated Share Repurchase Program, Authorized Amount							$ 350.0
Shares repurchased	$ 44.0	$ 19.0	$ 50.1	$ 50.0		$ 100.1
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 287.0				$ 287.0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	10,700,000				10,700,000
Treasury Stock [Member]
Share Repurchases [Line Items]
Treasury Stock, Shares, Acquired	2,133,116	3,500,000	1,451,171	1,500,000	5,607,003	2,970,674
Shares repurchased	$ 44.0	$ 89.0	$ 50.1	$ 50.0	$ 133.0
Additional Paid-in Capital [Member]
Share Repurchases [Line Items]
Treasury Stock, Shares, Acquired					2,607,172
Shares repurchased		$ (70.0)			$ 70.0