Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 49.2	$ 34.2	$ 89.9	$ 108.6
Level 3
Gain (Loss) on Securities [Line Items]
Net unrealized investment gains (losses)	$ 31.3	$ 24.6	$ 82.6	$ 76.0