Summary of Significant Accounting Policies - Stockholder's Equity - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 31, 2015	Apr. 30, 2015
Class of Stock [Line Items]
Authorized amount from Board of Directors for share repurchase		$ 500,000,000
Cost of shares repurchased		$ 85,400,000	$ 34,700,000	$ 115,000,000
Number of shares repurchased (in shares)		2,825,400	2,070,600
Common stock, shares authorized		400,000,000.0	400,000,000.0		400,000,000	200,000,000
Common stock, par value (in dollars per share)		$ 0.01			$ 0.01	$ 1.00
Subsequent event
Class of Stock [Line Items]
Shares repurchased and settled in subsequent month	$ 6,000,000