Derivative Financial Instruments (Schedule of Marketing Derivative Contracts Volume and Price) (Details) - Oil index swap contracts	Mar. 31, 2018 bbl / d $ / bbl
Louisiana Light Sweet | Second Quarter
Schedule of Marketing Derivative Contracts Volume and Price [Line Items]
Volume, barrels per day	10,000
Price differential, dollars per barrel | $ / bbl	3.18
Louisiana Light Sweet | Third Quarter
Schedule of Marketing Derivative Contracts Volume and Price [Line Items]
Volume, barrels per day	6,739
Price differential, dollars per barrel | $ / bbl	3.18
Louisiana Light Sweet | Index Swap Contracts for June 2018 Through August 2018
Schedule of Marketing Derivative Contracts Volume and Price [Line Items]
Volume, barrels per day	10,000
Magellan East Houston | Second Quarter
Schedule of Marketing Derivative Contracts Volume and Price [Line Items]
Volume, barrels per day	8,659
Price differential, dollars per barrel | $ / bbl	3.29
Magellan East Houston | Third Quarter
Schedule of Marketing Derivative Contracts Volume and Price [Line Items]
Volume, barrels per day	2,022
Price differential, dollars per barrel | $ / bbl	3.30