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Major Customers and Concentration of Credit Risk (Details) - First Major Customer - MPC	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues And Other Income]
Concentration Risk [Line Items]
Concentration risk, percentage	46.00%	36.00%	41.00%
Sales Revenue, Net [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	48.00%	37.00%	39.00%

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