Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Investments	$ 535,848	$ 575,148
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	27,124	17,752
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	9,330	9,741
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	112,258	90,320
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	72,790	60,081
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	215,821	175,088
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 168,553	$ 132,038