Summary of Significant Accounting Policies - Stockholder's Equity (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount	$ 500,000,000		$ 500,000,000
Treasury Stock, Shares, Acquired	1,356,484		4,327,158	1,942,200
Treasury Stock, Value, Acquired, Cost Method	$ 50,000,000		$ 150,100,000	$ 52,400,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 350,000,000		$ 350,000,000
Shares_repurchased_settled_in_subsequent_month		$ 6,000,000