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, 2021	$ 15,216.2	$ 4.0	$ 3,274.8	$ 13,644.3	$ (2,036.6)	$ 329.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,145.8	0.0	0.0	1,141.6	0.0	4.2
Exercise of stock options	5.8	0.0	5.8
Stock issued under employee incentive plans	11.4		11.4
Cash paid for shares withheld for taxes	(33.0)	0.0	(33.0)	0.0	0.0	0.0
Stock-based compensation expense	23.7		23.7
Cash dividends declared	(80.1)			(80.1)
Repurchases of common stock	(278.2)				(278.2)
Change of ownership interest in Forestar	1.8					1.8
Ending Balances at Dec. 31, 2021	16,013.4	4.0	3,282.7	14,705.8	(2,314.8)	335.7
Beginning Balances at Sep. 30, 2021	15,216.2	4.0	3,274.8	13,644.3	(2,036.6)	329.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,247.4
Cash paid for shares withheld for taxes	(61.8)
Ending Balances at Jun. 30, 2022	18,435.5	4.0	3,317.7	17,631.6	(2,890.8)	373.0
Beginning Balances at Dec. 31, 2021	16,013.4	4.0	3,282.7	14,705.8	(2,314.8)	335.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,442.3	0.0	0.0	1,436.3	0.0	6.0
Exercise of stock options	0.1	0.0	0.1
Stock issued under employee incentive plans	4.9		4.9
Cash paid for shares withheld for taxes	(28.7)	0.0	(28.7)	0.0	0.0	0.0
Stock-based compensation expense	30.9		30.9
Cash dividends declared	(79.1)			(79.1)
Repurchases of common stock	(266.0)				(266.0)
Change of ownership interest in Forestar	0.0		(1.2)			1.2
Ending Balances at Mar. 31, 2022	17,117.8	4.0	3,288.7	16,063.0	(2,580.8)	342.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,659.3	0.0	0.0	1,647.8	0.0	11.5
Exercise of stock options	0.6	0.0	0.6
Stock issued under employee incentive plans	4.1		4.1
Cash paid for shares withheld for taxes	(0.1)	0.0	(0.1)	0.0	0.0	0.0
Stock-based compensation expense	24.8		24.8
Cash dividends declared	(79.2)			(79.2)
Repurchases of common stock	(310.0)				(310.0)
Change of ownership interest in Forestar	0.0		(0.4)			0.4
Ending Balances at Jun. 30, 2022	18,435.5	4.0	3,317.7	17,631.6	(2,890.8)	373.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Noncontrolling Interest, Period Increase (Decrease)	18.2					18.2
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	3,269.7
Cash paid for shares withheld for taxes	(55.9)
Ending Balances at Jun. 30, 2023	22,082.3	4.0	3,394.7	22,164.4	(3,906.7)	425.9
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,351.7	0.0	0.0	1,335.1	0.0	16.6
Exercise of stock options	5.7	0.0	5.7
Stock issued under employee incentive plans	2.0		2.0
Cash paid for shares withheld for taxes	(0.1)	0.0	(0.1)	0.0	0.0	0.0
Stock-based compensation expense	29.3		29.3
Cash dividends declared	(85.2)			(85.2)
Repurchases of common stock	(342.9)				(342.9)
Change of ownership interest in Forestar	0.0		(0.2)			0.2
Ending Balances at Jun. 30, 2023	$ 22,082.3	$ 4.0	$ 3,394.7	$ 22,164.4	$ (3,906.7)	$ 425.9