Summary of Significant Accounting Policies - Stockholder's Equity (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 16, 2018
Class of Stock [Line Items]
Authorized amount from Board of Directors for share repurchase			$ 500,000,000
Cost of shares repurchased	$ (150,100,000)	$ (500,100,000)	$ (85,400,000)	$ (34,700,000)
Accelerated Share Repurchase Program, Authorized Amount					$ 350,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		70,000,000
Shares_repurchased_settled_in_subsequent_month		$ 6,000,000
Number of shares repurchased (in shares)	4,327,158	17,206,643	2,825,400
Common stock, shares authorized		400,000,000	400,000,000
Common stock, par value (in dollars per share)		$ 0.01
Treasury Stock [Member]
Class of Stock [Line Items]
Cost of shares repurchased		$ 430,100,000	$ (85,400,000)	$ (34,700,000)
Number of shares repurchased (in shares)		17,200,000	2,800,000	2,100,000
Minimum [Member]
Class of Stock [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		$ 40,000,000