Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Total investments	$ 183,500,000	$ 160,100,000
Investments in private equity funds percentage	57.00%	75.00%
Unfunded commitments	$ 0
Non Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Fair value, adjusted upward		$ 500,000
Fair value, adjusted downward	1,000,000.0
Total investments	84,500,000	45,500,000
Seed Capital Investments [Member]
Schedule of Investments [Line Items]
Total investments	21,200,000
Seed Capital Investments [Member] | Mutual Fund [Member]
Schedule of Investments [Line Items]
Total investments	10,000,000.0
Partnership Interests in Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	39,800,000	76,700,000
Investments, fair value	35,700,000	72,100,000
Equity method investments, fair value	$ 4,100,000	$ 4,600,000