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Related Party Transaction Gain (Loss) From Sale Excess Allowance Oil

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Related Party Transactions - Additional Information (Details) $ in Millions		3 Months Ended						6 Months Ended		12 Months Ended
	Nov. 03, 2014 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) term	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018	Sep. 30, 2018 credit_facility
Related Party Transaction [Line Items]
Statute of limitations expirations, number of days	60 days
Distributions to holders of incentive distribution rights waived					$ 17.0	$ 17.0
Gain (loss) from sale of excess allowance oil					1.0		$ 1.0	$ 3.0	$ 2.0
Health and life insurance costs							2.0	3.0	3.0
Defined contribution benefit plan costs					1.0		1.0	1.0	2.0
Reimbursement of cost and expenses					3.0		2.0	10.0	6.0
Operations and maintenance – related parties					$ 16.0		13.0	$ 30.0	26.0
Transportation Services Operating Leases
Related Party Transaction [Line Items]
Initial term					10 years			10 years
Transportation Services Operating Leases, Five Year Terms
Related Party Transaction [Line Items]
Initial term					10 years			10 years
Number of additional terms \| term								2
Additional term					5 years			5 years
Transportation Services Operating Leases, One Year Terms
Related Party Transaction [Line Items]
Initial term					10 years			10 years
Number of additional terms \| term								10
Additional term					1 year			1 year
SPLC
Related Party Transaction [Line Items]
Distributions to holders of incentive distribution rights waived					$ 17.0
General insurance expense					$ 5.0		$ 3.0	$ 9.0	$ 7.0
Shell Treasury Center West Inc
Related Party Transaction [Line Items]
Number of revolving credit facilities \| credit_facility												5
Refinery Gas Pipeline
Related Party Transaction [Line Items]
Reimbursement of cost and expenses								$ 0.6
Five Year Fixed Facility \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								5 years
Five Year Fixed Facility \| Shell Treasury Center West Inc \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								5 years
Five Year Revolver Due December 2022 \| Shell Treasury Center West Inc \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								5 years
Seven Year Fixed Facility \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								7 years
Seven Year Fixed Facility \| Shell Treasury Center West Inc \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								7 years
Ten Year Fixed Facility \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								10 years
Ten Year Fixed Facility \| Shell Treasury Center West Inc \| Revolving Credit Facility
Related Party Transaction [Line Items]
Debt instrument term								10 years
Shell Pipeline Company L P
Related Party Transaction [Line Items]
Payment of general and administrative fee	$ 11.0
Odyssey Pipeline L.L.C. (“Odyssey”)
Related Party Transaction [Line Items]
Noncontrolling interest					29.00%			29.00%			29.00%
Zydeco Pipeline Company LLC (“Zydeco”)
Related Party Transaction [Line Items]
Noncontrolling interest					7.50%			7.50%			7.50%
Scenario, Forecast
Related Party Transaction [Line Items]
Distributions to holders of incentive distribution rights waived				$ 16.0						$ 50.0
Scenario, Forecast \| SPLC
Related Party Transaction [Line Items]
Distributions to holders of incentive distribution rights waived		$ 50.0	$ 16.0	$ 17.0