Commitments and Contingencies (Details 2) bbl in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) bbl
Fixed Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	3,148
Sale commitments, Value | $	$ 178,629
Floating Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	12,152
Sale commitments, Value | $	$ 574,335
Fixed Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	1,461
Purchase commitments, Value | $	$ 83,545
Floating Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	10,915
Purchase commitments, Value | $	$ 641,934