Derivative Instruments (Derivative instruments recorded on the balance sheet at fair value - Non-printing) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair value of derivative instruments
Derivative assets liabilities fair value net	$ (68,947)	$ (13,126)
Oil roll swaps
Fair value of derivative instruments
Derivative assets liabilities fair value net	(73)
Natural Gas Basis Swaps
Fair value of derivative instruments
Derivative assets liabilities fair value net	$ 172

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Name:	us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant