Investments - Summary of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Total investments	$ 193.6	$ 183.5
Non Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Total investments	84.5	84.5
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Total investments	39.3	38.0
Partnership Interests in Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	37.7	39.8
Seed Capital Investments [Member]
Schedule of Investments [Line Items]
Total investments	$ 32.1	$ 21.2