Price risk management activities (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012 MMBbls
Fair Value of Outstanding Derivatives Contracts	$ 155,744	[1],[2]	$ 290,788	[1],[2]
March 2011 Swap
Beginning Date	Apr. 01, 2011	[1]
Ending Date	Feb. 28, 2013	[1]
Price Per Unit			104.55
Volumes Per Month			1,000	[1]
Fair Value of Outstanding Derivatives Contracts		[1],[2]	41,019	[1],[2]
November 2011 Swap
Beginning Date	Dec. 01, 2011	[1]
Ending Date	Nov. 30, 2014	[1]
Price Per Unit			93.50
Volumes Per Month			2,000	[1]
Fair Value of Outstanding Derivatives Contracts		[1],[2]	44,942	[1],[2]
February 2012 Swap
Beginning Date	Mar. 01, 2012	[1]
Ending Date	Feb. 28, 2014	[1]
Price Per Unit			106.50
Volumes Per Month			1,000	[1]
Fair Value of Outstanding Derivatives Contracts		[1],[2]	204,827	[1],[2]
February 2013 Swap
Beginning Date	Mar. 01, 2013	[1]
Ending Date	Nov. 01, 2014	[1]
Price Per Unit	93.50
Volumes Per Month	2,000	[1]
Fair Value of Outstanding Derivatives Contracts	48,570	[1],[2]		[1],[2]
February 2013 Swap 2
Beginning Date	Mar. 01, 2013	[1]
Ending Date	Feb. 01, 2014	[1]
Price Per Unit	106.50
Volumes Per Month	1,000	[1]
Fair Value of Outstanding Derivatives Contracts	$ 107,174	[1],[2]		[1],[2]

[1] These crude oil hedges were entered into on a per barrel delivered price basis, using the NYMEX - West Texas Intermediate Index, with settlement for each calendar month occuring following the expiration date, as determined by the contracts.

[2] The fair value of the Company's outstanding transactions is presented on the balance sheet by counterparty. Currently all of our derivatives are with the same counterparty. The balance is shown as current or long-term based on our estimates of the amounts that will be due in the relevant time periods at currently predicted price levels. Amounts in parentheses indicate liabilities.