Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets
Cash	$ 29,366	$ 23,726
Accounts receivable, net
Oil and gas sales	41,124	29,684
Joint interest owners	29,263	21,876
Other	1,848	4,590
Other current assets	5,156	1,088
Commodity derivative assets	18,053	17,648
Total current assets	124,810	98,612
Oil and gas properties, net, at cost under the successful efforts method	1,046,131	1,007,344
Other property, plant and equipment, net	2,764	3,398
Commodity derivative assets	34,540	25,199
Other assets	16,182	16,133
Total assets	1,224,427	1,150,686
Current liabilities
Trade accounts payable	60,248	38,036
Oil and gas sales payable	56,569	45,860
Accrued liabilities	5,122	3,873
Deferred tax liabilities	68	61
Asset retirement obligations	174	174
Commodity derivative liabilities	2,383	4,035
Total current liabilities	124,564	92,039
Long-term debt	605,000	610,000
Deferred revenue	15,000
Commodity derivative liabilities	88	7,657
Asset retirement obligations	10,023	9,332
Deferred tax liabilities	2,086	1,876
Total liabilities	756,761	720,904
Commitments and contingencies (Note 8)
Members' equity
Common units; 4,500,000 authorized and issued	70,224	65,043
Management units; 3,194,444 authorized and issued	19,014	14,228
Total members' equity	467,666	429,782
Total liabilities and members' equity	1,224,427	1,150,686
Class A preferred units
Members' equity
Preferred units	222,376	205,970
Class B preferred units
Members' equity
Preferred units	23,408	21,681
Class C preferred units
Members' equity
Preferred units	$ 132,644	$ 122,860