CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
CONSOLIDATED BALANCE SHEETS
Accounts receivable, trade, allowance (in dollars)	$ 1,135	$ 985
Common units, units issued	42,313,731	37,308,402
General partner units, interest (as a percent)	2.00%	2.00%
General partner units, equivalent units issued	858,583	757,722
General partner units, equivalent units outstanding	858,583	757,722
Treasury units, common units	43,630	33,811