Common Stock and Stock-Based Compensation Stockholders Equity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Nov. 16, 2018	Dec. 31, 2017
Share Repurchases [Line Items]
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	$ 100.9	$ 44.0	$ 19.0	$ 50.0	$ 50.1	$ 50.0		$ 150.1
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 186.1						$ 186.1
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	5,500,000						5,500,000
Share-based Payment Arrangement, Cash Used to Settle Award							$ 9.0
Additional Paid-in Capital [Member]
Share Repurchases [Line Items]
Treasury Stock, Shares, Acquired							2,607,172
Shares repurchased	$ 8.6		$ (70.0)				$ 70.0
Treasury Stock [Member]
Share Repurchases [Line Items]
Treasury Stock, Shares, Acquired	5,171,489	2,100,000	3,500,000	1,356,484	1,500,000	1,500,000	10,778,492	4,327,158
Shares repurchased	$ 100.9	$ 44.0	$ 89.0	$ 50.0	$ 50.1	$ 50.0	$ 233.9