Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts	$ 634	$ 470
Common stock par value (in dollars per share)	$ 0.001	$ 0.001
Common stock authorized (in shares)	135,000,000	135,000,000
Common stock issued (in shares)	84,209,056	83,855,824
Common stock outstanding (in shares)	83,996,817	83,643,585
Treasury stock (in shares)	212,239	212,239
Convertible Preferred Stock
Convertible preferred stock par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock authorized (in shares)	5,000,000	5,000,000
Convertible preferred stock issued (in shares)	0	0
Convertible preferred stock outstanding (in shares)	0	0

For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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