Fair Value Measurements - Schedule of Classification of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis and Investments Measured at NAV (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 658,532	$ 531,478
Total Derivative Assets	536	1,395
Total investments measured at fair value	659,068	532,873
Securities sold, not yet purchased	1,176	12,894
Total Derivative Liabilities	314,485	236,273
Total of Liabilities Measured at Fair Value	315,661	249,167
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	602,390	494,619
Securities sold, not yet purchased	1,176	12,894
Total of Liabilities Measured at Fair Value	1,176	12,894
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Derivative Assets	536	1,395
Total investments measured at fair value	536	1,395
Total Derivative Liabilities	314,485	236,273
Total of Liabilities Measured at Fair Value	314,485	236,273
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	3,157	2,971
NAV [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	52,985	33,888
Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	99,987	100,000
Debt [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	99,987	100,000
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	37,365	48,521
Equities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	35,694	46,921
Equities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,671	1,600
Alternative Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	34,264	16,581
Alternative Investment Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	17,411	15,731
Alternative Investment Funds [Member] | NAV [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	16,853	850
Debt Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	123,554	113,579
Debt Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	123,549	113,574
Debt Funds [Member] | NAV [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	5	5
Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	325,795	218,435
Equity Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	325,749	218,393
Equity Funds [Member] | NAV [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	46	42
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	37,567	34,362
Private Equity Funds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,486	1,371
Private Equity Funds [Member] | NAV [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 36,081	$ 32,991