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Equity Securities (Narrative) (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Schedule Of Investments [Line Items]          
Net (losses) gains on equity securities $ (29.4) $ (49.8) $ 20.5 $ (255.4)  
Net unrealized gains (losses) on equity securities (32.2) (33.2) 19.0 (231.4)  
Cash proceeds from sale of equity securities     61.1 230.8  
Offerpad Inc [Member]          
Schedule Of Investments [Line Items]          
Payment to purchase additional shares of common stock     25.0    
Investment cost 110.0   110.0   $ 85.0
Investment fair value 50.0   50.0   14.8
Non Marketable Equity Securities          
Schedule Of Investments [Line Items]          
Unrealized losses on equity securities 122.4 119.9      
Unrealized losses on equity securities     162.0 116.4  
Cumulative unrealized gains 241.8   241.8   241.8
Cumulative unrealized losses 315.2   315.2   $ 153.2
Investment In Insurance Business          
Schedule Of Investments [Line Items]          
Realized gain on securities       51.1  
Cash proceeds from sale of equity securities       63.0  
Common Stock | Offerpad Inc [Member]          
Schedule Of Investments [Line Items]          
Net unrealized gains (losses) on equity securities $ (16.4) $ (31.2) $ 10.2 $ (166.8)