UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 2,524	$ 2,453
Accumulated depreciation (in dollars)	120,119	$ 123,599
Convertible Preferred Stock | Series 4 Convertible Preferred Stock
Preferred Stock, Liquidation Preference, Value	$ 103,379