Concentration of Risk (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Glencore Energy UK Ltd [Member]	Dec. 31, 2011 Glencore Energy UK Ltd [Member]	Dec. 31, 2010 Glencore Energy UK Ltd [Member]	Dec. 31, 2012 Shell [Member]		Dec. 31, 2011 Shell [Member]
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)		39.00%	24.00%	17.00%	17.00%	[1]	17.00%	[1]
Percentage of total oil, gas & NGL sales (in hundredths)		31.00%	16.00%	11.00%	14.00%	[1]	12.00%	[1]
Concentration of credit risk [Abstract]
Short term deposits original maturities at time of purchase, description	three months or less
Accounts receivables, majority of accounts payment terms, description	30 days or less

[1] Includes sales to both Shell Trading (US) Company and Shell International Trading and Shipping Limited.