Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Investments	$ 421,727	$ 459,422
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	23,456	32,441
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	13,804	13,080
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	86,294	74,597
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	48,374	37,869
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	186,034	188,268
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 119,648	$ 128,219