Senior Debt - Senior Debt (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016	Feb. 28, 2015
Debt Instrument [Line Items]
Senior Debt, Outstanding Principal	$ 1,200,000,000	$ 1,200,000,000
Senior Debt, Outstanding Unamortized Debt Costs	9,617,000	11,400,000
Senior Debt, Outstanding Carrying Value	1,190,383,000	1,188,600,000
Lazard Group 4.25% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 500,000,000
Senior Debt, Maturity Date	Nov. 14, 2020
Senior Debt, Annual Interest Rate	4.25%
Senior Debt, Outstanding Principal	$ 500,000,000	500,000,000
Senior Debt, Outstanding Unamortized Debt Costs	2,647,000	3,569,000
Senior Debt, Outstanding Carrying Value	497,353,000	496,431,000
Lazard Group 3.75% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 400,000,000			$ 400,000,000
Senior Debt, Maturity Date	Feb. 13, 2025
Senior Debt, Annual Interest Rate	3.75%			3.75%
Senior Debt, Outstanding Principal	$ 400,000,000	400,000,000
Senior Debt, Outstanding Unamortized Debt Costs	3,361,000	3,833,000
Senior Debt, Outstanding Carrying Value	396,639,000	396,167,000
Lazard Group 3.625% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 300,000,000		$ 300,000,000
Senior Debt, Maturity Date	Mar. 01, 2027
Senior Debt, Annual Interest Rate	3.625%		3.625%
Senior Debt, Outstanding Principal	$ 300,000,000	300,000,000
Senior Debt, Outstanding Unamortized Debt Costs	3,609,000	3,998,000
Senior Debt, Outstanding Carrying Value	$ 296,391,000	$ 296,002,000