Consolidated Statements of Total Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock, Common	Noncontrolling Interest [Member]
Beginning Balances at Sep. 30, 2022	$ 19,785.6	$ 4.0	$ 3,349.5	$ 19,185.3	$ (3,142.5)	$ 389.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	968.3	0.0	0.0	958.7	0.0	9.6
Exercise of stock options	2.6	0.0	2.6
Stock issued under employee incentive plans	2.9		2.9
Cash paid for shares withheld for taxes	(25.7)	0.0	(25.7)	0.0	0.0	0.0
Stock-based compensation expense	22.9		22.9
Cash dividends declared	(86.1)			(86.1)
Repurchases of common stock	(118.1)				(118.1)
Change of ownership interest in Forestar	0.0		(0.2)			0.2
Ending Balances at Dec. 31, 2022	20,552.4	4.0	3,352.0	20,057.9	(3,260.6)	399.1
Beginning Balances at Sep. 30, 2022	19,785.6	4.0	3,349.5	19,185.3	(3,142.5)	389.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,917.9
Cash paid for shares withheld for taxes	(55.8)
Ending Balances at Mar. 31, 2023	21,121.8	4.0	3,358.0	20,914.5	(3,563.8)	409.1
Beginning Balances at Dec. 31, 2022	20,552.4	4.0	3,352.0	20,057.9	(3,260.6)	399.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	949.6	0.0	0.0	942.2	0.0	7.4
Exercise of stock options	5.6	0.0	5.6
Stock issued under employee incentive plans	4.7		4.7
Cash paid for shares withheld for taxes	(30.1)	0.0	(30.1)	0.0	0.0	0.0
Stock-based compensation expense	28.4		28.4
Cash dividends declared	(85.6)			(85.6)
Repurchases of common stock	(303.2)				(303.2)
Change of ownership interest in Forestar	0.0		(2.6)			2.6
Ending Balances at Mar. 31, 2023	21,121.8	4.0	3,358.0	20,914.5	(3,563.8)	409.1
Beginning Balances at Sep. 30, 2023	23,137.9	4.0	3,432.2	23,589.8	(4,329.8)	441.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	955.7	0.0	0.0	947.4	0.0	8.3
Exercise of stock options	1.6	0.0	1.6
Stock issued under employee incentive plans	3.1		3.1
Cash paid for shares withheld for taxes	(37.5)	0.0	(37.5)	0.0	0.0	0.0
Stock-based compensation expense	40.9		40.9
Cash dividends declared	(99.9)			(99.9)
Repurchases of common stock	(398.3)				(398.3)
Change of ownership interest in Forestar	0.0		(0.1)			0.1
Ending Balances at Dec. 31, 2023	23,603.5	4.0	3,440.2	24,437.3	(4,728.1)	450.1
Beginning Balances at Sep. 30, 2023	23,137.9	4.0	3,432.2	23,589.8	(4,329.8)	441.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	2,139.4
Cash paid for shares withheld for taxes	(81.6)
Cash dividends declared				(199.1)
Ending Balances at Mar. 31, 2024	24,296.4	4.0	3,431.6	25,510.2	(5,130.3)	480.9
Beginning Balances at Dec. 31, 2023	23,603.5	4.0	3,440.2	24,437.3	(4,728.1)	450.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,183.7	0.0	0.0	1,172.1	0.0	11.6
Exercise of stock options	3.6	0.0	3.6
Stock issued under employee incentive plans	6.9		6.9
Cash paid for shares withheld for taxes	(44.1)	0.0	(44.1)	0.0	0.0	0.0
Stock-based compensation expense	25.0		25.0
Cash dividends declared	(99.2)			(99.2)
Repurchases of common stock	(402.2)				(402.2)
Change of ownership interest in Forestar	19.2					19.2
Ending Balances at Mar. 31, 2024	$ 24,296.4	$ 4.0	$ 3,431.6	$ 25,510.2	$ (5,130.3)	$ 480.9