Summary of Significant Accounting Policies - Accounts Receivable and Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Accounts receivable, gross	$ 6,453	$ 4,133
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance Beginning of the Year	2,912	3,133	$ 2,511
Additions	9,712	6,704	11,237
Write-offs Net of Recoveries	(8,387)	(6,925)	(10,615)
Balance at End of Year	$ 4,237	$ 2,912	$ 3,133