Annual report pursuant to Section 13 and 15(d)

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

v3.10.0.1
Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Changes in Standardized Measure      
Changes in Standardized Measure, beginning of year $ 740.6 $ 478.3 $ 613.9
Sales and transfers of oil and gas produced, net of production costs (398.1) (315.3) (218.6)
Net changes in price, net of future production costs 571.5 288.0 (275.2)
Extensions and discoveries, net of future production and development costs 53.6 119.3  
Changes in estimated future development costs (114.7) (38.9) (32.5)
Previously estimated development costs incurred 48.4 102.8 114.5
Revisions of quantity estimates 307.6 106.4 190.1
Accretion of discount 50.5 30.2 52.6
Net change in income taxes (133.4) (54.7)  
Purchases of reserves in-place 27.8    
Sales of reserves in-place (54.1)    
Changes in production rates due to timing and other (32.7) 24.5 33.5
Net increase (decrease) in standardized measure 326.4 262.3 (135.6)
Changes in Standardized measure, end of year $ 1,067.0 $ 740.6 $ 478.3