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Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Goodwill [Line Items] |  |  |  |
Goodwill impairment |  | $ 0.0 | $ 7.2 | $ 9.8
Homebuilding [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill impairment | $ 7.2 | 0.0 | 7.2 | $ 9.8
Goodwill | 80.0 | 80.0 | 80.0 |
Homebuilding [Member] | East [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 21.8 | 21.8 | 21.8 |
Homebuilding [Member] | Midwest [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 0.0 | 0.0 | 0.0 |
Homebuilding [Member] | Southeast [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 40.1 | 40.1 | 40.1 |
Homebuilding [Member] | South Central [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 15.9 | 15.9 | 15.9 |
Homebuilding [Member] | Southwest [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | 0.0 | 0.0 | 0.0 |
Homebuilding [Member] | West [Member] |  |  |  |
Goodwill [Line Items] |  |  |  |
Goodwill | $ 2.2 | $ 2.2 | $ 2.2 |

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- Definition Amount of loss from the write-down of an asset representing the 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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