Segment Information - Concentrations of Revenue (Details) - Product Concentration Risk - Sales Revenue	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Percentage of Crude Oil Sales | BP
Concentration Risk [Line Items]
Concentration risk, percentage	31.00%	15.00%
Percentage of Crude Oil Sales | Shell
Concentration Risk [Line Items]
Concentration risk, percentage	22.00%	22.00%	24.00%
Percentage of Crude Oil Sales | Glencore Energy UK Ltd
Concentration Risk [Line Items]
Concentration risk, percentage			22.00%
Percentage of Total Oil, NGL & Gas Sales | BP
Concentration Risk [Line Items]
Concentration risk, percentage	17.00%	10.00%
Percentage of Total Oil, NGL & Gas Sales | Shell
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	13.00%	13.00%
Percentage of Total Oil, NGL & Gas Sales | Glencore Energy UK Ltd
Concentration Risk [Line Items]
Concentration risk, percentage			12.00%