Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Trade accounts receivable, allowances for doubtful accounts	$ 1,973,000	$ 1,496,000	$ 600,000	$ 329,000
Intangible assets, accumulated amortization	$ 7,425,000	$ 3,826,000
Partners' capital:
Common units issued and outstanding	33,186,155	33,142,114
Subordinated units issued and outstanding	0	6,273,970