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)

v3.6.0.2
Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Changes in Standardized Measure      
Changes in Standardized Measure, beginning of year $ 613.9 $ 1,702.8 $ 1,674.6
Sales and transfers of oil and gas produced, net of production costs (218.6) (289.1) (650.9)
Net changes in price, net of future production costs (275.2) (1,455.6) (278.6)
Extensions and discoveries, net of future production and development costs   65.3 309.6
Changes in estimated future development costs (32.5) (8.5) (56.4)
Previously estimated development costs incurred 114.5 158.9 263.1
Revisions of quantity estimates 190.1 137.9 118.6
Accretion of discount 52.6 150.6 180.6
Net change in income taxes   600.8 (11.4)
Purchases of reserves in-place   6.0 86.7
Sales of reserves in-place   (401.4)  
Changes in production rates due to timing and other 33.5 (53.8) 66.9
Net increase (decrease) in standardized measure (135.6) (1,088.9) 28.2
Changes in Standardized measure, end of year $ 478.3 $ 613.9 $ 1,702.8