Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Total investments	$ 505,194	$ 427,186
Investments, at fair value	504,664	426,630
Securities sold, not yet purchased, at fair value (included in "other liabilities")	4,419	7,338
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	530	556
Debt [Member]
Schedule of Investments [Line Items]
Total investments	74,583	6
Investments, at fair value	74,583	6
Equities [Member]
Schedule of Investments [Line Items]
Total investments	39,818	45,257
Investments, at fair value	39,818	45,257
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	26,097	20,993
Investments, at fair value	26,097	20,993
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	87,529	84,077
Investments, at fair value	87,529	84,077
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	209,379	199,618
Investments, at fair value	209,379	199,618
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	67,258	76,679
Investments, at fair value	67,258	76,679
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 390,263	$ 381,367