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, 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, 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