Condensed Consolidated Statements of Changes in Owners' Equity (USD $) In Thousands	Total	Common Unitholders	General Partner Interest
Beginning Balance at Dec. 31, 2009	$ 547,431	$ 548,160	$ (729)
Conversion of 80,534 and 84,842 vested phantom units in 2011 and 2010, respectively	2,580	2,580
Proceeds from public equity offering, net of underwriters discount and offering costs of $308 and $154 in 2011 and 2010, respectively	92,616	92,616
Contributions from general partner	1,977		1,977
Distributions	(43,433)	(38,284)	(5,149)
Equity-based compensation	827	827
Net income	62,404	61,156	1,248
Ending Balance at Jun. 30, 2010	664,402	667,055	(2,653)
Beginning Balance at Dec. 31, 2010	773,947	779,327	(5,380)
Conversion of 80,534 and 84,842 vested phantom units in 2011 and 2010, respectively	3,508	3,508
Proceeds from public equity offering, net of underwriters discount and offering costs of $308 and $154 in 2011 and 2010, respectively	146,800	146,800
Contributions from general partner	3,191		3,191
Distributions	(55,973)	(49,291)	(6,682)
Equity-based compensation	2,428	2,428
Net income	5,174	5,071	103
Ending Balance at Jun. 30, 2011	$ 879,075	$ 887,843	$ (8,768)