Investments - Company's Investments and Securities Sold, Not Yet Purchased (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Total investments	$ 459,422	$ 541,911
Equity method	222	8,917
Investments, at fair value	458,744	478,109
Securities sold, not yet purchased, at fair value (included in "other liabilities")	4,482	3,239
Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Total investments	456	54,885
Debt [Member]
Schedule of Investments [Line Items]
Total investments		535
Investments, at fair value		535
Equities [Member]
Schedule of Investments [Line Items]
Total investments	41,017	44,834
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Total investments	32,441	67,600
Investments, at fair value	32,441	67,600
Debt Funds [Member]
Schedule of Investments [Line Items]
Total investments	74,597	67,134
Investments, at fair value	74,597	67,134
Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	188,268	197,787
Investments, at fair value	188,268	197,787
Private Equity Funds [Member]
Schedule of Investments [Line Items]
Total investments	122,421	100,219
Investments, at fair value	122,421	100,219
Funds Total [Member]
Schedule of Investments [Line Items]
Total investments	$ 417,727	$ 432,740