Investments - Company's Investments and Securities Sold, Not Yet Purchased (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Investments	$ 427,186	$ 459,422
Alternative Investment Funds [Member]
Schedule of Investments [Line Items]
Investments	20,993	32,441
Alternative Investment Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	11,213	13,080
Debt Funds [Member]
Schedule of Investments [Line Items]
Investments	84,077	74,597
Debt Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	48,391	37,869
Equity Funds [Member]
Schedule of Investments [Line Items]
Investments	199,618	188,268
Equity Funds [Member] | Lazard Fund Interests [Member]
Schedule of Investments [Line Items]
Investments	$ 131,893	$ 128,219