Investments - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Total investments	$ 175,800,000	$ 190,500,000
Investments in private equity funds percentage	80.00%
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 upward	500,000
Total investments	45,500,000	45,000,000.0
Partnership Interests in Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	95,000,000.0	$ 102,100,000
Investments, fair value	90,300,000
Equity method investments, fair value	4,700,000
Other Private Equity Fund [Member]
Schedule of Investments [Line Items]
Investments, fair value	$ 1,700,000