Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 1,754	$ 6,291
Other assets:
Patents, trademarks, and other intangible assets, accumulated amortization	$ 79,264	$ 57,663
Equity:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	150,000,000
Common stock, shares issued	118,515,797	117,521,493
Treasury stock, shares held	2,638,093	2,536,421