Note 2 - Summary of Significant Accounting Policies (Details Textual) shares in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax, Total	$ 0	$ 0
Revenue from Contract with Customer, Including Assessed Tax	7,765,314	6,125,308
Unrecognized Tax Benefits, Ending Balance	$ 0	$ 0
Weighted Average Number of Shares Outstanding, Diluted, Total \| shares	0	0
Current Year Inflation Rate	4.08%
Change In Present Value, Percent	3.25%
Domestic Tax Authority [Member] \| Internal Revenue Service (IRS) [Member]
Open Tax Year	2015 2016 2017
Customer Concentration Risk [Member] \| Sales Revenue, Net [Member]
Number of Major Customers	2	2
Concentration Risk, Percentage	31.00%	35.00%
Oil and Gas [Member]
Contract with Customer, Asset, Net, Total	$ 346,711	$ 283,626
Oil and Condensate [Member]
Revenue from Contract with Customer, Including Assessed Tax	5,109,773	3,572,443
Natural Gas, US Regulated [Member]
Revenue from Contract with Customer, Including Assessed Tax	2,387,612	2,346,273
Miscellaneous Oil and Gas Sales [Member]
Revenue from Contract with Customer, Including Assessed Tax	$ 267,929	$ 206,592

The percentage by which the liability is increased annually for the change in its present value.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Tax year that remains open to examination under enacted tax laws, in CCYY format.

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