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Equity Securities (Narrative) (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Dec. 31, 2022
Schedule Of Investments [Line Items]          
Net (losses) gains on equity securities $ 37.5 $ (140.5) $ 49.9 $ (205.6)  
Net unrealized gains (losses) on equity securities 39.5 (133.4) 51.2 (198.2)  
Carrying amount of investment 8,377.5   8,377.5   $ 8,987.2
Cash proceeds from sale of equity securities     32.4 156.8  
Offerpad Inc [Member]          
Schedule Of Investments [Line Items]          
Payment to purchase additional shares of common stock     25.0    
Carrying amount of investment 66.3   66.3   $ 14.8
Non Marketable Equity Securities          
Schedule Of Investments [Line Items]          
Net unrealized gains on equity securities   15.0      
Impairment charges on equity securities 22.6   35.5    
Unrealized losses on equity securities 22.6   39.7 11.1  
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 $ 25.9 $ 91.6 $ 26.5 $ 135.6