Concentration of Risk (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
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
Glencore Energy UK Ltd [Member]
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	32.00%		34.00%		39.00%
Percentage of total oil, gas & NGL sales (in hundredths)	22.00%		25.00%		31.00%
Shell [Member]
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	15.00%	[1]	17.00%	[1]	17.00%	[1]
Percentage of total oil, gas & NGL sales (in hundredths)	10.00%	[1]	13.00%	[1]	14.00%	[1]

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