Basis of presentation and significant accounting policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
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	$ 57,647,000		$ 48,445,000
Joint operations	16,629,000	[1]	30,925,000	[1]
Other	3,042,000		4,470,000
Total	77,318,000		83,840,000
Allowance for doubtful accounts of accounts receivable for joint operations	700,000		100,000
Other current liabilities
Accrued interest payable	25,885,000		26,106,000
Lease operating expense payable	10,637,000		9,766,000
Prepaid drilling liability	1,393,000		2,916,000
Production taxes payable	1,303,000		2,121,000
Current portion of asset retirement obligations	265,000		385,000
Other accrued liabilities	16,037,000		2,782,000
Total other current liabilities	$ 55,520,000		$ 44,076,000

[1] Accounts receivable for joint operations are presented net of an allowance for doubtful accounts of $0.7 million and $0.1 million as of December 31, 2013 and 2012, respectively.