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Related Party Transactions (Details) $ in Millions	3 Months Ended		6 Months Ended
Related Party Transactions (Details) $ in Millions	Mar. 31, 2020 USD ($) Lot	Mar. 31, 2019 USD ($) Lot	Mar. 31, 2020 USD ($) Lot	Mar. 31, 2019 USD ($) Lot	Sep. 30, 2019 USD ($) Lot
Related Party Transaction [Line Items]
Number of Units in Real Estate Property \| Lot	52,300		52,300		38,300
Number of Lots Sold \| Lot	1,951	548	4,373	1,066
Real estate	$ 1,199.2		$ 1,199.2		$ 1,028.9
D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Selling, General and Administrative Expenses from Transactions with Related Party			2.6	$ 1.0
Related Party Transaction, Expenses from Transactions with Related Party			1.0	0.6
Due to Related Parties, Current	4.8		4.8		$ 2.2
Deferred Revenue, Period Increase (Decrease)	0.8	$ 2.1	(0.6)	(1.5)
Related Party Transaction, Purchases from Related Party	$ 1.5	$ 0.3	$ 2.9	$ 0.8
D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Number of Lots Sold \| Lot	1,906	453	4,296	908
Revenue from Related Parties	$ 151.2	$ 37.7	$ 366.8	$ 70.2
Under Contract [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Number of Units in Real Estate Property \| Lot	14,200		14,200		12,800
Related Party Deposit Liabilities	$ 98.4		$ 98.4		$ 88.7
Related Party Transaction, Purchase Obligation from Parent	$ 1,033.1		$ 1,033.1		$ 953.8
Right of First Offer [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Number of Units in Real Estate Property \| Lot	14,400		14,400		10,600
Land [Member]
Related Party Transaction [Line Items]
Real estate	$ 28.2		$ 28.2		$ 17.1
Other Expense [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	8.2	0.7	13.3	3.7
Deposits [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Amounts of Transaction	$ 5.5	$ 4.7	$ 16.2	$ 16.8
Land [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Number of Lots Sold \| Lot	0	0	36	0
Revenue from Related Parties	$ 0.0	$ 0.0	$ 7.2	$ 0.0
Minimum [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Rate			12.00%
Maximum [Member] \| D.R. Horton, Inc. [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Rate			16.00%
Real Estate [Member] \| Residential Real Estate [Member]
Related Party Transaction [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 156.4	$ 49.3	$ 373.5	$ 84.0