Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Investments	$ 575,148	$ 427,186
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	17,752	20,993
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	9,741	11,213
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	90,320	84,077
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	60,081	48,391
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	175,088	199,618
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 132,038	$ 131,893