Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Senior Debt - Senior Debt (Detail) - USD ($)	9 Months Ended
Senior Debt - Senior Debt (Detail) - USD ($)	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Senior Debt, Outstanding Principal	$ 1,700,000,000		$ 1,450,000,000
Senior Debt, Outstanding Unamortized Debt Costs	21,079,000		15,740,000
Senior Debt, Outstanding Carrying Value	1,678,921,000		1,434,260,000
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			250,000,000
Senior Debt, Outstanding Unamortized Debt Costs			863,000
Senior Debt, Outstanding Carrying Value			249,137,000
Lazard Group 3.75% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 400,000,000
Senior Debt, Maturity Date	Feb. 13, 2025
Senior Debt, Annual Interest Rate	3.75%
Senior Debt, Outstanding Principal	$ 400,000,000		400,000,000
Senior Debt, Outstanding Unamortized Debt Costs	2,534,000		2,888,000
Senior Debt, Outstanding Carrying Value	397,466,000		397,112,000
Lazard Group 3.625% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 300,000,000
Senior Debt, Maturity Date	Mar. 01, 2027
Senior Debt, Annual Interest Rate	3.625%
Senior Debt, Outstanding Principal	$ 300,000,000		300,000,000
Senior Debt, Outstanding Unamortized Debt Costs	2,920,000		3,215,000
Senior Debt, Outstanding Carrying Value	297,080,000		296,785,000
Lazard Group 4.50% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 500,000,000			$ 500,000,000
Senior Debt, Maturity Date	Sep. 19, 2028
Senior Debt, Annual Interest Rate	4.50%
Senior Debt, Outstanding Principal	$ 500,000,000		500,000,000
Senior Debt, Outstanding Unamortized Debt Costs	8,038,000		8,774,000
Senior Debt, Outstanding Carrying Value	491,962,000		$ 491,226,000
Lazard Group 4.375% Senior Notes [Member]
Debt Instrument [Line Items]
Senior Debt, Initial Principal Amount	$ 500,000,000	$ 500,000,000
Senior Debt, Maturity Date	Mar. 11, 2029
Senior Debt, Annual Interest Rate	4.375%	4.375%
Senior Debt, Outstanding Principal	$ 500,000,000
Senior Debt, Outstanding Unamortized Debt Costs	7,587,000
Senior Debt, Outstanding Carrying Value	$ 492,413,000