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Investment Securities - Investment gains (losses) for Level 3 (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Level 3 | Other long-term investments        
Debt and Equity Securities, FV-NI [Line Items]        
Total net unrealized investment gains on other long-term investments held at the end of the period, pre-tax $ 14.4 $ (10.2) $ 51.4 $ 17.0