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Summary of Significant Accounting Policies - Business Acquisitions (Details) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Oct. 05, 2017 | Sep. 02, 2016 Lot Home
Business Acquisition [Line Items] |  |  |  |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired |  |  | $ 4.1 | $ 82.2 | $ 70.9 |  |
Wilson Parker Homes [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business Acquisition, Date of Acquisition Agreement |  | Sep. 02, 2016 |  |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired |  | $ 91.9 |  |  |  |  |
Business Acquisition, Number of Homes Acquired | Home |  |  |  |  |  |  | 380
Business Acquisition, Number of Finished Lots Acquired | Lot |  |  |  |  |  |  | 490
Business Acquisition, Number of Lots Under Option Contracts | Lot |  |  |  |  |  |  | 1,850
Business Acquisition, Sales Order Backlog Acquired (Homes) | Home |  |  |  |  |  |  | 308
Goodwill, Acquired During Period |  | $ 0.0 |  |  |  |  |
Subsequent Event [Member] | Forestar Group [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business Acquisition, Date of Acquisition Agreement | Oct. 05, 2017 |  |  |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired | $ 558.3 |  |  |  |  |  |
Business Acquisition, Percentage of Voting Interests Acquired |  |  |  |  |  | 75.00% |

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