Commitments and Contingencies - Legal Claims and Insurance (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Rollforward of reserves for legal claims
Reserves for legal claims, beginning of period			$ 434.7	$ 408.1
Increase in reserves			33.6	23.3
Payments			(26.1)	(12.3)
Reserves for legal claims, end of period	$ 442.2	$ 419.1	442.2	419.1
Forestar Group [Member]
Rollforward of reserves for legal claims
Increase (Decrease) in Prepaid Expenses, Other	$ 8.2	$ 0.7	$ 13.3	$ 3.7

Amount of increase (decrease) of consideration paid in advance for other costs that provide economic benefits in future periods.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
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Amount of loss contingency liability.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

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Amount of increase (decrease) in loss contingency liability.

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Amount of cash outflow reducing loss contingency liability.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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