Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 1,380	$ 1,286
Other assets:
Patents, trademarks, and other intangible assets, accumulated amortization	$ 74,149	$ 71,114
Equity:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	128,349,334	118,515,797
Treasury stock, shares held	2,015,775	2,638,093