Variable Interest Entity Arrangements (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Date Involvement Began	Apr. 03, 2018
Superior Pipeline Company, L.L.C. [Member]
Variable Interest Entity [Line Items]
Date Involvement Began	Apr. 03, 2018
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 houses the power to direct the activities that most significantly impact Superior's operating performance. The MSA is a separate variable interest. Unit through the MSA has the power to direct Superior’s most significant activities; reciprocally the equity investors lack the power to direct the activities that most significantly impact the entity’s economic performance. Because of this, Unit is considered the primary beneficiary.
Lack of Recourse	Superior's creditors have no recourse to our general credit.
Superior Pipeline Company, L.L.C. [Member] | SPC Midstream Operating, L.L.C. [Member]
Variable Interest Entity [Line Items]
Monthly service fee	$ 250
Superior Pipeline Company, L.L.C. [Member] | SP Investor Holdings, LLC [Member]
Variable Interest Entity [Line Items]
Ownership percentage	50.00%