Notes Payable - Principal Amounts (Details) - USD ($) $ in Millions	Sep. 30, 2020	May 01, 2020	Oct. 01, 2019	Sep. 30, 2019
Summary of notes payable at principal amounts, net of unamortized discounts
Notes payable	$ 4,283.3			$ 3,399.4
Financial Services [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Notes payable	1,132.6			888.9
Forestar Consolidated [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	8.9			6.2
Summary of notes payable at principal amounts, net of unamortized discounts
Line of credit, amount outstanding	0.0			0.0
Notes payable	641.1			462.9
Home Building Consolidated
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	10.7			5.4
Summary of notes payable at principal amounts, net of unamortized discounts
Line of credit, amount outstanding	0.0			0.0
Line of Credit, Current	0.0			0.0
Notes payable	$ 2,509.6			2,047.6
Senior Note Member Thirty [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	4.00%
Notes payable	$ 0.0			499.6
Senior Note Member Thirty-One [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	2.55%
Notes payable	$ 399.8			398.9
Senior Note Member Twenty Five [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	4.375%
Notes payable	$ 349.2			348.8
Senior Note Twenty Seven [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	4.75%
Notes payable	$ 299.2			298.9
SeniorNoteTwentyEight [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	5.75%
Notes payable	$ 398.7			398.4
SeniorNoteFortyTwo [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	2.50%		2.50%
Notes payable	$ 496.5			0.0
SeniorNoteFortyFour | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	2.60%	2.60%
Notes payable	$ 495.1			0.0
Senior Note Member Forty [Member] | Forestar Consolidated [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	3.75%
Notes payable	$ 0.0			119.1
Senior Note Member Forty One [Member] | Forestar Consolidated [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	8.00%
Notes payable	$ 345.2			343.8
Senior Note Member Forty Three | Forestar Consolidated [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Senior notes, stated interest rate	5.00%
Notes payable	$ 295.9			0.0
Secured Debt [Member] | Home Building Consolidated
Summary of notes payable at principal amounts, net of unamortized discounts
Notes payable	71.1			103.0
Segment Reconciling Items [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Notes payable	0.0			2.4
Intersegment Eliminations [Member]
Summary of notes payable at principal amounts, net of unamortized discounts
Notes payable	$ 4.8			$ 0.0