Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Total investments	$ 354,952	$ 427,186
Investments, at fair value	354,403	426,630
Securities sold, not yet purchased, at fair value (included in "other liabilities")	4,274	7,338
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	549	556
Debt [Member]
Schedule of Investments [Line Items]
Total investments	7	6
Investments, at fair value	7	6
Equities [Member]
Schedule of Investments [Line Items]
Total investments	40,778	45,257
Investments, at fair value	40,778	45,257
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	17,547	20,993
Investments, at fair value	17,547	20,993
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	62,808	84,077
Investments, at fair value	62,808	84,077
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	157,693	199,618
Investments, at fair value	157,693	199,618
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	75,570	76,679
Investments, at fair value	75,570	76,679
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 313,618	$ 381,367