Investments - Unrealized Gains on Equity Securities (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2021 equity_position	Dec. 31, 2022 USD ($)
Gain (Loss) on Securities [Line Items]
Net losses recognized on equity securities	$ (3.0)	$ (5.2)	$ (15.9)	$ (102.4)
Less: Net realized gains (losses) related to sales of equity securities	0.1	(0.2)	(2.2)	20.0
Total fair value changes to equity securities held	(3.1)	(5.0)	(13.7)	(122.4)
Number of equity positions that went public | equity_position					4
Equity securities at fair value	256.5		256.5			$ 281.3
Four Equity Positions that Went Public
Gain (Loss) on Securities [Line Items]
Equity securities, unrealized gain	1.7	$ 0.6	6.2	$ 78.5
Equity securities at fair value	$ 3.4		$ 3.4