Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 34.2	$ 27.5	$ 108.6	$ 40.8
Level 3
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 24.6	$ 40.8	$ 76.0	$ 57.7