Investments - Summary of Investments (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Total investments	$ 175.5	$ 160.1
Partnership Interests in Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	61.2	76.7
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Total investments	29.8	37.9
Non Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Total investments	$ 84.5	$ 45.5