Basis of Presentation and Summary of Significant Accounting Policies - Supplemental Schedule of Noncash Investing and Financing Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental disclosure of cash flow information—cash paid during the period for:
Accrued purchases of property, equipment, and software	$ 2,195	$ 2,728	$ 851
Accretion of dividend equivalents	3,532	3,244	2,978
Vendor financed insurance premiums	$ 1,450	$ 2,082	$ 0