Commitments and Contingencies - Purchase and sales commitments (Details) bbl in Thousands, $ in Thousands	Mar. 31, 2017 USD ($) bbl
Fixed Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	5,073
Sale commitments, Value | $	$ 253,795
Floating Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) | bbl	18,365
Sale commitments, Value | $	$ 855,639
Fixed Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	4,158
Purchase commitments, Value | $	$ 204,181
Floating Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) | bbl	13,304
Purchase commitments, Value | $	$ 655,885