Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended							6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2018
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Stock Repurchase Program, Authorized Amount				$ 1,000.0
Treasury Stock, Shares, Acquired								7,000,000.0
Payments for Repurchase of Common Stock								$ 360.4	$ 216.2
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 535.3						$ 535.3
Dividends, Common Stock [Abstract]
Divedends declared (in dollars per sh		$ 0.175	$ 0.175	$ 0.15	$ 0.15	$ 0.15	$ 0.15
Cash dividends paid per common share (in dollars per share)		$ 0.175	$ 0.175	$ 0.15	$ 0.15	$ 0.15	$ 0.15
Common Stock, Shares, Issued		394,279,177		392,172,821				394,279,177
Subsequent Event [Member]
Dividends, Common Stock [Abstract]
Divedends declared (in dollars per sh	$ 0.175
Forestar Group [Member]
Class of Stock [Line Items]
Equity Securities Registered, Value										$ 500.0
Dividends, Common Stock [Abstract]
Common Stock, Shares, Issued				6,000,000.0
Shares Issued, Price Per Share				$ 17.50
Common Stock Available for Issuance, Value Remaining		$ 394.3						$ 394.3