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Nature of operations and summary of significant accounting policies (Tables)
 3 Months Ended
Mar. 31, 2014
Accounting Policies [Abstract]  
Accounts receivable
Accounts receivable consisted of the following at March 31, 2014 and December 31, 2013: 
 
March 31,
2014
 
December 31,
2013
Joint interests
$
40,454

 
$
31,335

Accrued commodity sales
73,370

 
60,768

Derivative settlements
156

 
4,616

Other
2,913

 
1,392

Allowance for doubtful accounts
(506
)
 
(1,596
)
 
$
116,387

 
$
96,515
Inventories
Inventories at March 31, 2014 and December 31, 2013 consisted of the following: 
 
 
March 31,
2014
 
December 31,
2013
Equipment inventory
 
$
19,437

 
$
13,657

Commodities
 
3,347

 
3,186

Inventory valuation allowance
 
(706
)
 
(706
)
 
 
$
22,078

 
$
16,137