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Variable Interest Entity Arrangements (Narrative) (Details) - Superior Pipeline Company, L.L.C. [Member]
12 Months Ended
Dec. 31, 2020
USD ($)
Variable Interest Entity [Line Items]  
Date Involvement Began Apr. 03, 2018
Lack of Recourse Superior's creditors have no recourse to our general credit.
Methodology for Determining Whether Entity is Primary Beneficiary The two variable interests applicable to Unit include the 50% equity investment in Superior and the MSA. The MSA gives us the power to direct the activities that most significantly affect Superior's operating performance. The MSA is a separate variable interest. Under the MSA, Unit has the power to direct Superior’s most significant activities; reciprocally the equity investors lack the power to direct the activities that most affect the entity’s economic performance. Because of this, Unit is considered the primary beneficiary.
SPC Midstream Operating, L.L.C. [Member]  
Variable Interest Entity [Line Items]  
Monthly service fee $ 260,560
SP Investor Holdings, LLC [Member]  
Variable Interest Entity [Line Items]  
Ownership percentage 50.00%