Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Total investments	$ 421,727	$ 459,422
Equity method	222	222
Investments, at fair value	420,964	458,744
Securities sold, not yet purchased, at fair value (included in "other liabilities")	4,527	4,482
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	541	456
Debt [Member]
Schedule of Investments [Line Items]
Total investments	7
Investments, at fair value	7
Equities [Member]
Schedule of Investments [Line Items]
Total investments	46,137	41,017
Investments, at fair value	46,137	41,017
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	23,456	32,441
Investments, at fair value	23,456	32,441
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	86,294	74,597
Investments, at fair value	86,294	74,597
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	186,034	188,268
Investments, at fair value	186,034	188,268
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	79,036	122,421
Investments, at fair value	79,036	122,421
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 374,820	$ 417,727