Allowance for Doubtful Accounts Roll Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Balance, beginning of year	$ 1,829	$ 1,417	$ 1,844
Provision (net of recovery)	370	898	346
Amounts written off, net of recoveries	(242)	(486)	(773)
Balance, end of year	$ 1,957	$ 1,829	$ 1,417