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Goodwill - Schedule of Goodwill (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019
Goodwill [Roll Forward] |  |  |  |
Gross amount of goodwill |  |  | $ 102,300,000 | $ 102,300,000
Accumulated impairment losses |  |  | (86,900,000) | (37,200,000)
Goodwill, Beginning Balance |  |  | 15,400,000 | 65,100,000
Impairment losses | $ (49,700,000) | $ (37,200,000) |  | (49,700,000)
Gross amount of goodwill included in disposal |  |  | (7,100,000) |
Accumulated impairment losses included in disposal |  |  | 2,200,000 |
Gross amount of goodwill |  | 102,300,000 | 95,200,000 | 102,300,000
Accumulated impairment losses |  | (37,200,000) | (84,700,000) | (86,900,000)
Goodwill, Ending Balance |  | 65,100,000 | 10,500,000 | 15,400,000
Materials and Construction Segment |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Gross amount of goodwill |  |  | 93,600,000 | 93,600,000
Accumulated impairment losses |  |  | (86,900,000) | (37,200,000)
Goodwill, Beginning Balance |  |  | 6,700,000 | 56,400,000
Impairment losses |  |  |  | (49,700,000)
Gross amount of goodwill included in disposal |  |  | (7,100,000) |
Accumulated impairment losses included in disposal |  |  | 2,200,000 |
Gross amount of goodwill |  | 93,600,000 | 86,500,000 | 93,600,000
Accumulated impairment losses |  | (37,200,000) | (84,700,000) | (86,900,000)
Goodwill, Ending Balance |  | 56,400,000 | 1,800,000 | 6,700,000
Commercial property |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Gross amount of goodwill |  |  | 8,700,000 | 8,700,000
Accumulated impairment losses |  |  | 0 | 0
Goodwill, Beginning Balance |  |  | 8,700,000 | 8,700,000
Impairment losses |  |  |  | 0
Gross amount of goodwill included in disposal |  |  | 0 |
Accumulated impairment losses included in disposal |  |  | 0 |
Gross amount of goodwill |  | 8,700,000 | 8,700,000 | 8,700,000
Accumulated impairment losses |  | 0 | 0 | 0
Goodwill, Ending Balance |  | $ 8,700,000 | 8,700,000 | $ 8,700,000
Land Operations |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Ending Balance |  |  | $ 0 |

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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