Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Investments	$ 505,194	$ 427,186
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	26,097	20,993
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	11,898	11,213
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	87,529	84,077
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	57,444	48,391
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	209,379	199,618
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 147,862	$ 131,893