Summary of Significant Accounting Policies - Stockholder Equity (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Authorized stock repurchase		$ 500,000,000		$ 500,000,000
Shares repurchased, shares		1,451,171	1,942,200	2,970,674	1,942,200
Shares repurchased, amount		$ 50,100,000	$ 52,400,000	$ 100,100,000	$ 52,400,000
Remaining authorized repurchase amount		$ 400,000,000		$ 400,000,000
Subsequent Event
Class of Stock [Line Items]
Shares_repurchased_settled_in_subsequent_month	$ 3,800,000