Consolidated Statements of Total Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Noncontrolling Interest [Member]
Beginning Balances at Sep. 30, 2019	$ 10,295.1	$ 3.9	$ 3,179.1	$ 7,640.1	$ (802.2)	$ 274.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	432.5	0.0	0.0	431.3	0.0	1.2
Exercise of stock options	4.1	0.0	4.1
Stock issued under employee incentive plans			0.0
Cash paid for shares withheld for taxes	(17.3)	0.0	(17.3)	0.0	0.0	0.0
Stock-based compensation expense	16.6		16.6
Cash dividends declared	(64.6)			(64.6)
Repurchases of common stock	(163.1)				(163.1)
Distributions to noncontrolling interests	(0.4)					(0.4)
Change of ownership interest in Forestar	0.0		(0.5)			0.5
Ending Balances at Dec. 31, 2019	10,502.9	3.9	3,182.0	8,006.8	(965.3)	275.5
Beginning Balances at Sep. 30, 2019	10,295.1	3.9	3,179.1	7,640.1	(802.2)	274.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	916.4
Cash paid for shares withheld for taxes	(38.1)
Ending Balances at Mar. 31, 2020	10,734.8	3.9	3,191.3	8,425.4	(1,162.6)	276.8
Beginning Balances at Dec. 31, 2019	10,502.9	3.9	3,182.0	8,006.8	(965.3)	275.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	484.0	0.0	0.0	482.7	0.0	1.3
Exercise of stock options	6.2	0.0	6.2
Stock issued under employee incentive plans	2.6		2.6
Cash paid for shares withheld for taxes	(20.8)	0.0	(20.8)	0.0	0.0	0.0
Stock-based compensation expense	21.3		21.3
Cash dividends declared	(64.1)			(64.1)
Repurchases of common stock	(197.3)				(197.3)
Ending Balances at Mar. 31, 2020	10,734.8	3.9	3,191.3	8,425.4	(1,162.6)	276.8
Beginning Balances at Sep. 30, 2020	12,121.5	3.9	3,240.9	9,757.8	(1,162.6)	281.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	795.2	0.0	0.0	791.8	0.0	3.4
Exercise of stock options	0.9	0.0	0.9
Stock issued under employee incentive plans	0.1	0.1	0.0
Cash paid for shares withheld for taxes	(26.3)	0.0	(26.3)	0.0	0.0	0.0
Stock-based compensation expense	21.7		21.7
Cash dividends declared	(72.9)			(72.9)
Repurchases of common stock	(69.8)				(69.8)
Distributions to noncontrolling interests	(0.1)					(0.1)
Change of ownership interest in Forestar	0.0		(0.3)			0.3
Ending Balances at Dec. 31, 2020	12,770.3	4.0	3,236.9	10,476.7	(1,232.4)	285.1
Beginning Balances at Sep. 30, 2020	12,121.5	3.9	3,240.9	9,757.8	(1,162.6)	281.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,729.1
Cash paid for shares withheld for taxes	(82.9)
Ending Balances at Mar. 31, 2021	13,276.0	4.0	3,208.4	11,333.5	(1,582.8)	312.9
Beginning Balances at Dec. 31, 2020	12,770.3	4.0	3,236.9	10,476.7	(1,232.4)	285.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	933.9	0.0	0.0	929.5	0.0	4.4
Exercise of stock options	1.4	0.0	1.4
Stock issued under employee incentive plans	3.2		3.2
Cash paid for shares withheld for taxes	(56.6)	0.0	(56.6)	0.0	0.0	0.0
Stock-based compensation expense	25.4		25.4
Cash dividends declared	(72.7)			(72.7)
Repurchases of common stock	(350.4)				(350.4)
Change of ownership interest in Forestar	21.5		(1.9)			23.4
Ending Balances at Mar. 31, 2021	$ 13,276.0	$ 4.0	$ 3,208.4	$ 11,333.5	$ (1,582.8)	$ 312.9