The amount of asset impairment charges and mine closure costs during the period.

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Amount recognized as an operating expense or loss during the period to reduce the carrying amount of coal and mineral rights.

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Amount recognized as an operating expense or loss during the period to reduce the carrying amount of deferred development costs.

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Amount recognized as an operating expense or loss during the period to reduce the carrying amount of parts and supplies.

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Represents information pertaining to number of corporate staff accepted voluntary separation package.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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The charge against earnings resulting from the write down of long lived assets other than goodwill due to the difference between the carrying value and lower fair value.

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Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

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Asset impairment and restructuring (Details) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
Asset impairment and restructuring (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 item	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Coal lands and mineral rights					$ 33,197
Plant and equipment					43,197
Deferred development					76,807
Parts and supplies					8,639
Equity investment					36,167
Severance costs related to voluntary separation plan					13,373
Other					10,000
Asset impairment charges					221,380
Letters of credit outstanding	$ 20,000				20,000
Number of corporate staff accepted voluntary separation package \| item			254
Knight Hawk
Equity investment		$ 36,200		$ 36,200	$ 36,167
Prior Equity Investment
Asset impairment charges	10,000
Coal Creek Mine
Asset impairment charges	$ 32,800
PRB \| Coal Creek Mine
Asset impairment charges		51,800		51,800
Other Thermal \| Viper Mine
Asset impairment charges		33,500		33,500
Other Thermal \| West Elk Mine
Asset impairment charges		$ 41,600		$ 41,600