Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Investments	$ 523,366	$ 531,995
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	32,610	16,581
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	9,886	9,881
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	134,527	113,579
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	100,858	78,360
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	280,853	218,435
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 228,353	$ 170,897