Debt - Schedule of Notes Payable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
4.75% Senior Notes, due July 15, 2022	$ 375,000	$ 375,000
Unamortized issuance discount on notes payable	(300)	(359)
Total notes payable	$ 374,700	$ 374,641