Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - 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) - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]      
Future cash flows $ 3,354,168 $ 3,043,450 $ 4,667,678
Future development and abandonment costs (1,165,025) (877,660) (719,898)
Future production costs (924,167) (941,243) (880,427)
Future production taxes (69,447) (58,169) (71,229)
Future income taxes (14,545) (2,648) (693,154)
Future net cash flows 1,180,984 1,163,730 2,302,970
10% discount to reflect timing of cash flows (492,944) (399,399) (875,803)
Standardized measure of discounted future net cash flows 688,040 764,331 1,427,167
Investment in Grizzly Oil Sands ULC      
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]      
Future cash flows 0 0 754,720
Future development and abandonment costs 0 0 (205,242)
Future production costs 0 0 (291,988)
Future production taxes 0 0 0
Future income taxes 0 0 (11,250)
Future net cash flows 0 0 246,240
10% discount to reflect timing of cash flows (152,494)
Standardized measure of discounted future net cash flows $ 0 $ 0 $ 93,746