Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable
Balance, beginning of year	$ 3,080	$ 3,600	$ 3,323
Additions to expense	2,434	614	533
Write-offs	(1,392)	(1,102)	(318)
Recoveries	5		89
Other	22	(32)	(27)
Balance, end of year	$ 4,149	$ 3,080	$ 3,600