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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) Mcf / bbl purchaser | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |
Accounts receivable, collection period | 30 days |  |
Allowance for doubtful accounts | $ 0 | $ 0 |
Conversion ratio, gas to barrels of oil | Mcf / bbl | 6 |  |
Capitalized costs of oil and natural gas properties excluded from amortization | $ 2,912,974,000 | 1,580,305,000 |
Gas imbalance receivable | 200,000 | 200,000 |
Investment in Grizzly Oil Sands ULC |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Equity method investment recognized impairment charges | $ 0 | $ 23,100,000 | $ 101,600,000
Minimum |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life | 3 years |  |
Minimum | Restricted stock |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Vesting period | 1 year |  |
Maximum |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life | 30 years |  |
Maximum | Restricted stock |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Vesting period | 4 years |  |
Customer concentration risk | Accounts receivable |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Concentration risk, number of purchasers | purchaser | 3 |  |

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