Investments - Summary of Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Net Investment Income [Line Items]
Total investments	$ 177.4	$ 184.7
Non Marketable Equity Securities
Net Investment Income [Line Items]
Total investments	124.1	124.0
Seed Capital Investments
Net Investment Income [Line Items]
Total investments	23.1	26.1
Marketable Equity Securities
Net Investment Income [Line Items]
Total investments	21.6	23.1
Partnership interests in private equity funds
Net Investment Income [Line Items]
Total investments	$ 8.6	$ 11.5