Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Total investments	$ 535,848	$ 575,148
Investments, at fair value	535,353	574,638
Securities sold, not yet purchased, at fair value (included in "other liabilities")	1,555	3,929
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	495	510
Debt [Member]
Schedule of Investments [Line Items]
Total investments	99,535	202,874
Investments, at fair value	99,535	202,874
Equities [Member]
Schedule of Investments [Line Items]
Total investments	38,114	32,261
Investments, at fair value	38,114	32,261
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	27,124	17,752
Investments, at fair value	27,124	17,752
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	112,258	90,320
Investments, at fair value	112,258	90,320
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	215,821	175,088
Investments, at fair value	215,821	175,088
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	42,501	56,343
Investments, at fair value	42,501	56,343
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 397,704	$ 339,503