Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 1,237	$ 1,349
Other assets:
Patents, trademarks, and other intangible assets, accumulated amortization	$ 33,212	$ 31,956
Equity:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	81,393,372	81,333,631
Treasury stock, shares held	2,491,187	2,478,084