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General Information (Unaudited) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jul. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2019
General Information [Abstract] |  |  |  |
Employee separation charges |  | $ 34 | $ 258 |
Production and delivery charges |  | 30 | 202 |
Depreciation, depletion, and amortization |  | $ 3 | 32 |
Selling, general, and administrative expense |  |  | 15 |
Mining exploration and research expense |  |  | $ 8 |
Expected long-term rate of return on plan assets | 2.40% |  |  | 3.40%
Discount rate used in calculating benefit obligation | 6.25% | 3.25% | 3.25% | 6.50%
Accumulated benefit obligation, decrease for curtailment during period |  |  | $ 184 |
Increase in plan assets during period |  |  | 103 |
Curtailment loss recognized |  | $ 4 |  |
Net funded status of defined benefit plan |  | $ 888 | $ 888 |

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