Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Investments	$ 426,948	$ 459,422
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	23,518	32,441
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	13,850	13,080
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	86,900	74,597
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	48,451	37,869
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	191,193	188,268
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 124,570	$ 128,219