Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Investments	$ 782,351	$ 658,532
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	39,371	34,264
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	15,254	11,128
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	140,053	123,554
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	107,207	90,758
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	415,056	325,795
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 351,548	$ 277,725