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Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Goodwill [Line Items] |  |  |
Goodwill impairment | $ 0.0 | $ 0.0 | $ 0.0
Goodwill | 163.5 | 163.5 |
Home Building [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 134.3 | 134.3 |
Home Building [Member] | East [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 26.4 | 26.4 |
Home Building [Member] | Midwest [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 49.7 | 49.7 |
Home Building [Member] | Southeast [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 40.1 | 40.1 |
Home Building [Member] | South Central [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 15.9 | 15.9 |
Home Building [Member] | Southwest [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 0.0 | 0.0 |
Home Building [Member] | West [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 2.2 | 2.2 |
Segment Reconciling Items [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | $ 29.2 | $ 29.2 |

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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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