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, 2017
Dec. 31, 2016
Dec. 31, 2015
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]      
Future cash flows $ 11,202,692 $ 3,354,168 $ 3,043,450
Future development and abandonment costs (3,005,217) (1,165,025) (877,660)
Future production costs (2,152,821) (924,167) (941,243)
Future production taxes (289,944) (69,447) (58,169)
Future income taxes (573,965) (14,545) (2,648)
Future net cash flows 5,180,745 1,180,984 1,163,730
10% discount to reflect timing of cash flows (2,537,181) (492,944) (399,399)
Standardized measure of discounted future net cash flows 2,643,564 688,040 764,331
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 0
Future development and abandonment costs 0 0 0
Future production costs 0 0 0
Future production taxes 0 0 0
Future income taxes 0 0 0
Future net cash flows 0 0 0
10% discount to reflect timing of cash flows
Standardized measure of discounted future net cash flows $ 0 $ 0 $ 0