Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Investments	$ 658,532	$ 531,995
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	34,264	16,581
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	11,128	9,881
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	123,554	113,579
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	90,758	78,360
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	325,795	218,435
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 277,725	$ 170,897