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Risk Management Activities - Market Risk - Commodity Price - Oil - Additional Information (Detail) l in Millions, $ in Millions, $ in Millions | 1 Months Ended | 2 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2019 T ppm l bbl | Jun. 30, 2017 ppm l bbl | May 31, 2017 T ppm l bbl | Jun. 30, 2017 ppm l bbl | Dec. 31, 2020 USD ($) | Dec. 31, 2020 AUD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2019 AUD ($) | Dec. 31, 2020 AUD ($) | Dec. 31, 2019 AUD ($)
Australia [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Marked-to-market value of the hedge |  |  |  |  | $ (5.1) |  | $ 1.0 |  | $ (6.6) | $ 1.4
Total realized gain/(loss) |  |  |  |  | (3.4) | $ (4.9) | 3.1 | $ 4.5 |  |
Australia [member] | Commodity price risk [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Unrealized Gain (loss) on hedge ineffectiveness |  |  |  |  | (5.5) | (8.0) | (0.8) | (1.1) |  |
Australia [member] | Commodity price risk [member] | Fair value hedges [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Marked-to-market value of the hedge |  |  |  |  | 27.3 |  | 78.3 |  | $ 35.5 | $ (111.5)
Total realized gain/(loss) |  |  |  |  | (201.4) | (292.2) | (113.4) | (163.0) |  |
Unrealized Gain (loss) on hedge ineffectiveness |  |  |  |  | 71.8 | $ 104.0 | (65.4) | $ (93.4) |  |
Australia [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Risk hedging number | ppm | 10 | 10 | 10 | 10 |  |  |  |  |  |
Volume of diesel hedged | l | 75.0 |  |  | 77.5 |  |  |  |  |  |
Average swap price | 74.0 | 61.2 | 61.2 |  |  |  |  |  |  |
Benchmark price per barrel at time transaction |  | 49.9 |  | 49.9 |  |  |  |  |  |
Australia [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] | Brent crude [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Benchmark price per barrel at time transaction | 57.4 | 49.9 |  | 49.9 |  |  |  |  |  |
Ghana [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Marked-to-market value of the hedge | $ |  |  |  |  | (10.0) |  | 0.1 |  |  |
Total realized gain/(loss) | $ |  |  |  |  | (6.8) |  | 5.4 |  |  |
Ghana [member] | Commodity price risk [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Average swap price | T | 575 |  | 457.2 |  |  |  |  |  |  |
Unrealized Gain (loss) on hedge ineffectiveness | $ |  |  |  |  | (10.1) |  | (2.9) |  |  |
Ghana [member] | Commodity price risk [member] | Fair value hedges [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Marked-to-market value of the hedge | $ |  |  |  |  | 0.0 |  | (36.4) |  |  |
Total realized gain/(loss) | $ |  |  |  |  | $ (114.5) |  | $ 2.3 |  |  |
Ghana [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Volume of diesel hedged | l | 123.2 | 125.8 | 125.8 |  |  |  |  |  |  |
Average swap price | 75.8 | 61.4 |  |  |  |  |  |  |  |
Benchmark price per barrel at time transaction |  | 49.8 |  | 49.8 |  |  |  |  |  |
Ghana [member] | Commodity price risk [member] | Fair value hedges [member] | Cash settled swap transaction contracts [member] | Brent crude [member] |  |  |  |  |  |  |  |  |  |
Disclosure of Financial Risk Management [line items] |  |  |  |  |  |  |  |  |  |
Benchmark price per barrel at time transaction | 59.2 | 49.8 |  | 49.8 |  |  |  |  |  |

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