Investments - Unrealized Gains on Equity Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Net losses recognized on equity securities	$ (9.8)	$ (54.0)	$ (12.9)	$ (97.2)
Less: Net realized (losses) gains related to sales of equity securities	(1.5)	8.4	(2.3)	20.2
Total fair value changes to equity securities held	(8.3)	(62.4)	(10.6)	(117.4)
Gain (Loss) on Securities [Line Items]
Equity securities at fair value	259.6		259.6		$ 281.3
Four Equity Positions that Went Public
Gain (Loss) on Securities [Line Items]
Equity securities, unrealized gain	2.2	$ 44.2	4.5	$ 77.9
Equity securities at fair value	$ 5.1		$ 5.1