Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details)

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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs $ (312,291) $ (486,185) $ (530,098)
Net changes in prices, production costs, and development costs (146,518) (1,412,181) 97,716
Acquisition of oil and gas reserves in place 0 83,340 14,266
Extensions and discoveries 186,909 262,895 790,533
Previously estimated development costs incurred during the period 176,218 117,540 68,227
Revisions of previous quantity estimates, less related production costs (38,448) (98,162) (37,801)
Accretion of discount 76,433 142,717 57,847
Net changes in income taxes (6,495) 412,240 (295,226)
Change in production rates and other (12,099) 314,960 683,237
Total change in standardized measure of discounted future net cash flows (76,291) (662,836) 848,701
Investment in Grizzly Oil Sands ULC      
Changes In Standardized Measure of Discontinued Future Net Cash Flows [Line Items]      
Sales and transfers of oil and gas produced, net of production costs 0 114 4,664
Net changes in prices, production costs, and development costs 0 0 (76,518)
Acquisition of oil and gas reserves in place 0 0 0
Extensions and discoveries 0 0 7,107
Previously estimated development costs incurred during the period 0 47 0
Revisions of previous quantity estimates, less related production costs 0 (103,282) 10,659
Accretion of discount 0 9,375 14,946
Net changes in income taxes 0 0 9,162
Change in production rates and other 0 0 (25,738)
Total change in standardized measure of discounted future net cash flows $ 0 $ (93,746) $ (55,718)