Summary of Significant Accounting Policies - Stockholder Equity (Details) - USD ($)	1 Months Ended	3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 500,000,000
Shares repurchased (in shares)		1,519,503	0
Treasury Stock, Value, Acquired		$ 50,000,000	$ 0
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 450,000,000
Subsequent Event
Class of Stock [Line Items]
Shares_repurchased_settled_in_subsequent_month	$ 6,700,000