Basis of presentation and significant accounting policies - Current assets and liabilities (Details) (USD $)	6 Months Ended
	Jun. 30, 2014 segment		Dec. 31, 2013
Basis of presentation
Number of operating segments	1
Accounts receivable
Term of accounts receivable to be considered as past due (in days)	30 days
Term of past due balances to be reviewed individually for collectability (in days)	90 days
Oil and natural gas sales	$ 68,476,000		$ 57,647,000
Joint operations, net	23,274,000	[1]	16,629,000	[1]
Other	3,970,000		3,042,000
Total	95,720,000		77,318,000
Allowance for doubtful accounts of accounts receivable for joint operations	700,000		700,000
Other current liabilities
Accrued interest payable	37,065,000		25,885,000
Accrued compensation and benefits	11,804,000		16,711,000
Lease operating expense payable	11,056,000		10,637,000
Performance unit awards	2,971,000		0
Asset retirement obligations	886,000		265,000
Prepaid drilling liability	851,000		1,393,000
Other accrued liabilities	13,374,000		17,340,000
Total other current liabilities	$ 78,007,000		$ 72,231,000

[1] Accounts receivable for joint operations are presented net of an allowance for doubtful accounts of $0.7 million as of June 30, 2014 and December 31, 2013.