Investments - Summary of Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Total investments	$ 184.7	$ 193.9
Non Marketable Equity Securities [Member]
Net Investment Income [Line Items]
Total investments	124.0	124.0
Marketable Equity Securities [Member]
Net Investment Income [Line Items]
Total investments	23.1	24.3
Seed Capital Investments [Member]
Net Investment Income [Line Items]
Total investments	26.1	30.6
Partnership interests in private equity funds
Net Investment Income [Line Items]
Total investments	$ 11.5	$ 15.0