Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 5,309	$ 4,072	$ 2,363
Provision for doubtful accounts	2,324	1,229	1,135
Amounts written off, net of recoveries	(1,927)	8	574
Balance at end of year	$ 5,706	$ 5,309	$ 4,072