Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Investments	$ 354,952	$ 427,186
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	17,547	20,993
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	8,294	11,213
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	62,808	84,077
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	32,190	48,391
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	157,693	199,618
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 89,491	$ 131,893