Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable
Balance, beginning of year	$ 4,149	$ 3,080	$ 3,600
Additions to expense	462	2,434	614
Write-offs	(1,659)	(1,392)	(1,102)
Recoveries		5
Other	163	22	(32)
Balance, end of year	$ 3,115	$ 4,149	$ 3,080