Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Total investments	$ 382,074	$ 459,422
Equity method	222	222
Investments, at fair value	381,369	458,744
Securities sold, not yet purchased, at fair value (included in "other liabilities")	4,293	4,482
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	483	456
Equities [Member]
Schedule of Investments [Line Items]
Total investments	42,962	41,017
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	31,046	32,441
Investments, at fair value	31,046	32,441
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	61,486	74,597
Investments, at fair value	61,486	74,597
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	135,926	188,268
Investments, at fair value	135,926	188,268
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	109,949	122,421
Investments, at fair value	109,949	122,421
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 338,407	$ 417,727