Concentration of Risk (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	30.00%	32.00%	34.00%
Percentage of total oil, gas & NGL sales (in hundredths)	18.00%	22.00%	25.00%
Shell
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	18.00%	15.00%	17.00%
Percentage of total oil, gas & NGL sales (in hundredths)	11.00%	10.00%	13.00%