Note 13 - Hedging Program and Derivatives (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Asset, Total	$ 20,129
Gain (Loss) on Sale of Derivatives	8,060	(1,849)	$ (18,028)
Commodity Contract [Member]
Derivative Asset, Total	15,100
Commodity Contract [Member] \| (Gain) Loss on Derivative Contracts [Member]
Gain (Loss) on Sale of Derivatives	8,100	(1,800)
Commodity Closed Contracts [Member] \| (Gain) Loss on Derivative Contracts [Member]
Gain (Loss) on Sale of Derivatives	(19,000)	2,500
Commodity Open Contracts [Member] \| (Gain) Loss on Derivative Contracts [Member]
Gain (Loss) on Sale of Derivatives	$ 27,100	$ (4,300)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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