Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Common stock, shares issued		394,741,349				392,172,821				394,741,349	392,172,821
Common stock, shares outstanding		363,999,982				368,431,454				363,999,982	368,431,454
Preferred stock, shares issued		0				0				0	0
Treasury Stock, Common, Shares										7,000,000.0
Payments for Repurchase of Common Stock										$ 360.4	$ 479.8	$ 127.5
Amount of stock repurchase authorization						$ 1,000.0					$ 1,000.0
Amount remaining under stock repurchase authorization		$ 535.3								$ 535.3
Dividends, Common Stock [Abstract]
Cash dividends declared per common share		$ 0.175	$ 0.175	$ 0.175	$ 0.175	$ 0.15	$ 0.15	$ 0.15	$ 0.15
Cash dividends paid per common share		$ 0.175	$ 0.175	$ 0.175	$ 0.175	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.70	$ 0.60	$ 0.50
Forestar Group [Member]
Class of Stock [Line Items]
Equity Securities Registered, Value												$ 500.0
Common Stock Available for Issuance, Value Remaining		$ 394.3								$ 394.3
Dividends, Common Stock [Abstract]
At-the-market Equity Offering Program, Common Stock Available for Issuance		100.0								100.0
At-the-market Equity Offering Program, Common Stock Issued		$ 0.0								$ 0.0
Subsequent Event [Member]
Dividends, Common Stock [Abstract]
Cash dividends declared per common share	$ 0.20