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]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		368,431,454
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		258,800
Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture		582,936
Divedends declared (in dollars per sh	$ 0.175	$ 0.175
Stock Repurchased and Retired During Period, Shares		(3,000,000)
Common Stock, Shares, Outstanding		366,273,190
Common Stock, Shares, Outstanding	363,999,982	363,999,982
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		42,950
Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture		604,947
Divedends declared (in dollars per sh	$ 0.20	$ 0.20
Stock Repurchased and Retired During Period, Shares		(1,000,000)
Common Stock, Shares, Outstanding	363,647,879	363,647,879