Notes Payable - Homebuilding Textuals (Details) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Oct. 01, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Notes payable		$ 3,399,400,000		$ 3,399,400,000		$ 3,203,500,000
Maturities of Long-term Debt [Abstract]
Maturities of notes payable in fiscal 2019		1,600,000,000		1,600,000,000
Maturities of notes payable in fiscal 2020		403,300,000		403,300,000
Maturities of notes payable in fiscal 2021		350,400,000		350,400,000
Maturities of notes payable in fiscal 2022		700,400,000		700,400,000
Maturities of notes payable in fiscal 2023		351,500,000		351,500,000
Letters of credit, amount outstanding		$ 171,700,000		$ 171,700,000
Percentage of Company Assets Owned by Entities that Guarantee Homebuilding Senior Notes and Revolving Credit Facility		79.00%		79.00%
Home Building [Member]
Debt Instrument [Line Items]
Notes payable		$ 2,047,600,000		$ 2,047,600,000		2,445,900,000
Maturities of Long-term Debt [Abstract]
Line of credit, current borrowing capacity		1,325,000,000		1,325,000,000
Line of credit, maximum borrowing capacity		$ 1,900,000,000		$ 1,900,000,000
Letter of Credit, Maximum Borrowing Capacity		50.00%		50.00%
Proceeds from Lines of Credit				$ 2,100,000,000
Repayments of Long-term Lines of Credit				2,100,000,000
Line of credit, amount outstanding		$ 0		0		0
Line of Credit Facility, Remaining Borrowing Capacity		1,200,000,000		1,200,000,000
Authorized repurchase of debt securities		500,000,000
Debt repurchase program, remaining authorized repurchase amount		500,000,000		500,000,000
Home Building [Member] | Revolving credit facility [Member]
Maturities of Long-term Debt [Abstract]
Letters of credit, amount outstanding		$ 141,200,000		$ 141,200,000
Senior Note Twenty Nine [Member] | Home Building [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate		3.75%		3.75%
Notes payable		$ 0		$ 0		$ 499,600,000
Maturities of Long-term Debt [Abstract]
Maturities of Senior Debt			$ 500,000,000
Senior Notes [Member] | Home Building [Member]
Maturities of Long-term Debt [Abstract]
Debt Instrument, Redemption Price, Percentage				100.00%
Senior Notes Subject to Repurchase Upon Change of Control and Ratings Downgrade [Member] | Home Building [Member]
Maturities of Long-term Debt [Abstract]
Debt Instrument, Redemption Price, Percentage				101.00%
Subsequent Event [Member] | Home Building [Member]
Maturities of Long-term Debt [Abstract]
Line of credit, current borrowing capacity					$ 1,590,000,000
Line of credit, maximum borrowing capacity					$ 2,500,000,000
Letter of Credit, Maximum Borrowing Capacity					100.00%
Subsequent Event [Member] | SeniorNoteFortyTwo [Member] | Home Building [Member]
Debt Instrument [Line Items]
Senior notes, stated interest rate					2.50%
Notes payable					$ 500,000,000
Maturities of Long-term Debt [Abstract]
Debt Instrument, Maturity Date	Oct. 15, 2024
Debt Instrument, Interest Rate, Effective Percentage					2.70%