Commitments and Contingencies - Purchase and sales commitments (Details) bbl in Thousands, $ in Thousands	Mar. 31, 2018 USD ($) bbl
Fixed Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	1,590
Sale commitments, Value | $	$ 99,470
Floating Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	10,884
Sale commitments, Value | $	$ 567,025
Fixed Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	1,360
Purchase commitments, Value | $	$ 85,555
Floating Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	8,366
Purchase commitments, Value | $	$ 531,648