Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Long-term debt, net	$ 512,176,000	$ 504,090,000
Current assets:
Trade accounts receivable, allowances for doubtful accounts	822,000	2,253,000
Intangible assets, accumulated amortization	$ 21,829,000	$ 18,666,000
Partners' capital:
Common units issued and outstanding	37,618,734	33,262,376