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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($) Mcf / bbl | Dec. 31, 2017 USD ($) | Dec. 31, 2016 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 (in Mcf of gas) | Mcf / bbl | 6 |  |
Capitalized costs of oil and natural gas properties excluded from amortization | $ 2,873,037,000 | 2,912,974,000 |
Unrecognized tax benefits | 0 |  |
Gas imbalance receivable |  | 0 |
Cash and cash equivalents | 52,297,000 | 99,557,000 |
Net cash used in investing activities | (643,091,000) | (2,474,168,000) | $ (720,582,000)
Investment in Grizzly Oil Sands ULC |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Equity method investment recognized impairment charges | $ 0 | 0 | 23,100,000
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 |  |
Accounting Standards Update 2016-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Lease liability | $ 110,000,000 |  |
Accounting Standards Update 2016-18 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Cash and cash equivalents | $ 0 |  | 185,000,000
Net cash used in investing activities |  | $ 185,000,000 | $ (185,000,000)

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