Accounts Receivable - Summary of changes in allowance for doubtful accounts (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 2,553	$ 10,605
Bad debt expense (reduction)	1,441	6,021
Accounts written-off	(105)	(14,101)
Recoveries	582	28
Ending balance	$ 4,471	$ 2,553