Equity Securities (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Investments [Line Items]
Realized and unrealized net gains (losses) on equity securities	$ 54.9	$ (262.5)
Net unrealized gains on equity securities	51.2	(237.2)
Cash proceeds from sale of equity securities	71.0	241.4	$ 171.8
Offerpad Inc [Member]
Schedule Of Investments [Line Items]
Net unrealized gains on equity securities	12.7	(190.9)
Payment to purchase additional shares of common stock	25.0
Investment cost	110.0	85.0
Investment fair value	52.5	14.8
Non Marketable Equity Securities
Schedule Of Investments [Line Items]
Cumulative unrealized losses and impairment charges	322.4	153.2
Cumulative unrealized gains	$ 243.3	241.8
Realized gain on securities		51.1
Cash proceeds from sale of equity securities		$ 63.0