Annual report pursuant to Section 13 and 15(d)

Note 20 - Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details)

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Note 20 - Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Changes in Standardized Measure      
Standardized measure, beginning of year $ 1,067.0 $ 740.6 $ 478.3
Sales and transfers of oil and gas produced, net of production costs (315.8) (398.1) (315.3)
Net changes in price, net of future production costs (376.4) 571.5 288.0
Extensions and discoveries, net of future production and development costs 27.0 53.6 119.3
Changes in estimated future development costs (6.0) (114.7) (38.9)
Previously estimated development costs incurred 19.3 48.4 102.8
Revisions of quantity estimates 116.4 307.6 106.4
Accretion of discount 107.4 50.5 30.2
Net change in income taxes (62.9) 133.4 54.7
Purchases of reserves in-place 298.3 27.8
Sales of reserves in-place (54.1)
Changes in production rates due to timing and other (13.2) (32.7) 24.5
Net (decrease) increase (80.1) 326.4 262.3
Standardized measure, end of year $ 986.9 $ 1,067.0 $ 740.6