Selected Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Data (Unaudited)
Revenue	$ 102,301,000	$ 99,324,000	$ 97,171,000	$ 88,697,000	$ 83,267,000	$ 84,437,000	$ 71,841,000	$ 68,729,000	$ 387,493,000	$ 308,274,000	$ 237,636,000
Gross profit	34,228,000	29,469,000	1,541,000	24,020,000	21,570,000	22,215,000	16,696,000	17,927,000
Net income (loss)	14,674,000	10,380,000	(19,050,000)	4,505,000	4,515,000	3,253,000	(1,938,000)	223,000	10,509,000	6,053,000	(23,333,000)
Income (loss) per common unit - basic (in dollars per unit)	$ 0.31	$ 0.21	$ (0.47)	$ 0.09	$ 0.10	$ 0.06	$ (0.08)	$ (0.01)	$ 0.14	$ 0.09	$ (0.90)
Income (loss) per subordinated unit - basic (in dollars per unit)					$ 0.10	$ 0.06	$ (0.08)	$ (0.01)		$ 0.09	$ (0.90)
Income (loss) per common unit - diluted (in dollars per unit)	$ 0.31	$ 0.21	$ (0.47)	$ 0.09	$ 0.10	$ 0.06	$ (0.08)	$ (0.01)	$ 0.14	$ 0.09	$ (0.90)
Income (loss) per subordinated unit - diluted (in dollars per unit)					$ 0.10	$ 0.06	$ (0.08)	$ (0.01)		$ 0.09	$ (0.90)
Long-lived asset impairment charge			$ 28,100,000