Significant Accounting Policies (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Balance at beginning of year	$ 1,028	$ 974	$ 878
Net charges to expense	192	711	685
Amounts recovered against the allowance	(523)	(657)	(589)
Balance at end of year	$ 697	$ 1,028	$ 974