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Commitments and Contingencies Commitments and Contingencies - Take or Pay (Details) - SemGas [Member] - Fractionation capacity [Member]
$ in Thousands
Mar. 31, 2018
USD ($)
Long-term Purchase Commitment [Line Items]  
Purchase Obligation, Remainder of the year $ 7,950
Purchase Obligation, Due in Second Year 9,567
Purchase Obligation, Due in Third Year 8,864
Purchase Obligation, Due in Fourth Year 7,175
Purchase Obligation, Due in Fifth Year 6,753
Purchase Obligation, Due after Fifth Year 2,791
Purchase Obligation $ 43,100