Concentration of Risk (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
BP
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	15.00%
Percentage of total oil, gas & NGL sales (in hundredths)	10.00%
Shell
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)	22.00%	24.00%	18.00%
Percentage of total oil, gas & NGL sales (in hundredths)	13.00%	13.00%	11.00%
Glencore Energy UK Ltd
Concentration Risk [Line Items]
Percentage of crude oil sales (in hundredths)		22.00%	30.00%
Percentage of total oil, gas & NGL sales (in hundredths)		12.00%	18.00%