Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 1,651	$ 2,583
Other assets:
Patents, trademarks, and other intangible assets, accumulated amortization	$ 82,634	$ 80,401
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	128,412,688	128,455,134
Treasury stock, shares held	2,785,981	2,717,569