Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 888	$ 2,253
Patents, trademarks, and other intangible assets, accumulated amortization	$ 21,089	$ 18,666
Partners' capital:
Common units issued and outstanding	35,495,660	33,262,376