Investments - Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Total net gains (losses) recognized during the period on equity securities	$ 300.1	$ 109.8
Less: Net gains (losses) recognized on equity securities sold during the period	5.2	19.3
Net holding period gains (losses) recognized during the period on equity securities held at period end	$ 294.9	$ 90.5