Summary of Significant Accounting Policies (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the allowance for doubtful accounts
Balance, beginning of year	$ 664	$ 709	$ 510
Charged to provision for doubtful accounts	113	558	367
Write-offs	(44)	(603)	(168)
Balance, end of year	$ 733	$ 664	$ 709