Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 470	$ 1,037
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, authorized (in shares)	135,000,000	100,000,000
Common stock, issued (in shares)	83,855,824	82,986,244
Common stock, outstanding (in shares)	83,643,585	82,774,005
Treasury stock (in shares)	212,239	212,239
Convertible Preferred Stock
Convertible preferred stock, par value (in USD 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

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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