Investments - Additional Information (Detail) - USD ($) shares in Millions		3 Months Ended
	Feb. 27, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Total investments		$ 175,500,000		$ 160,100,000
Investments in private equity funds percentage		74.00%		75.00%
Private equity fund termination date		2020
Private equity fund termination extension period		1 year
Unfunded commitments		$ 0
Non Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Fair value, adjusted downward		1,000,000.0
Fair value, adjusted upward			$ 400,000
Total investments		84,500,000		$ 45,500,000
Partnership Interests in Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments		61,200,000		76,700,000
Investments, fair value		57,100,000		72,100,000
Equity method investments, fair value		4,100,000		4,600,000
Other Private Equity Fund [Member]
Schedule of Investments [Line Items]
Investments, fair value		$ 1,600,000		$ 1,600,000
Series A Convertible Preferred Stock [Member] | Non Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Number of shares acquired	40
Investment acquired purchase price	$ 40,000,000.0