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Asset Retirement Obligations (ARO Activity) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Carrying amount of the asset retirement obligations, balance at beginning of period | $ 37,703 | $ 40,097
Liabilities incurred during the period | 91 | 189
Liabilities settled during the period | (9,017) | (1,667)
Accretion expense | 2,611 | 2,579
Revision of estimated cash flows | 3,145 | (3,495)
Carrying amount of the asset retirement obligations, balance at end of period | 36,859 | 37,703
Operating Segments | Coal Mining |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Carrying amount of the asset retirement obligations, balance at beginning of period | 20,396 | 22,589
Liabilities incurred during the period | 0 | 0
Liabilities settled during the period | (8,265) | (920)
Accretion expense | 1,260 | 1,504
Revision of estimated cash flows | 3,145 | (2,777)
Carrying amount of the asset retirement obligations, balance at end of period | 19,015 | 20,396
Operating Segments | NAMining |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Carrying amount of the asset retirement obligations, balance at beginning of period | 485 | 1,085
Liabilities incurred during the period | 91 | 189
Liabilities settled during the period | 0 | 0
Accretion expense | 28 | 31
Revision of estimated cash flows | 0 | (820)
Carrying amount of the asset retirement obligations, balance at end of period | 604 | 485
Unallocated Items |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Carrying amount of the asset retirement obligations, balance at beginning of period | 16,822 | 16,423
Liabilities incurred during the period | 0 | 0
Liabilities settled during the period | (752) | (747)
Accretion expense | 1,323 | 1,044
Revision of estimated cash flows | 0 | 102
Carrying amount of the asset retirement obligations, balance at end of period | 17,240 | $ 16,822
Centennial |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Revision of estimated cash flows | 2,326 |
Centennial | Operating Segments | Coal Mining |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Revision of estimated cash flows | 2,479 |
Centennial | Operating Segments | NAMining |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Revision of estimated cash flows | 0 |
Centennial | Unallocated Items |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Revision of estimated cash flows | $ (153) |

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 + Details
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- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 35 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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