Summary of Significant Accounting Policies Credit Risk (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Concentration Risk, Percentage	49.00%	59.00%	48.00%
Plains Marketing & Transportation [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	12.00%
Shell Trading Company [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage		14.00%	14.00%
Occidental Energy Marketing [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	16.00%	13.00%
Andeavor Logistics LP [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage		13.00%
BP Energy Company [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage		10.00%	10.00%
Plains Marketing LP [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage		10.00%
Valero Marketing And Supply Company [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage			10.00%