Senior Debt - Senior Debt (Parenthetical) (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Sep. 30, 2018
Lazard Group 4.375% Senior Notes [Member]
Debt Instrument [Line Items]
Senior debt, initial principal amount		$ 500,000,000	$ 500,000,000
Interest rate, payment terms			Interest on the 2029 Notes is payable semi-annually on March 11 and September 11 of each year, beginning September 11, 2019.
Senior notes interest rate		4.375%	4.375%
Original Maturity Date			Mar. 11, 2029
Effective interest rates of senior notes			4.54%
Lazard Group 4.25% Senior Notes [Member]
Debt Instrument [Line Items]
Senior debt, initial principal amount			$ 500,000,000
Senior notes interest rate			4.25%
Redemption of senior debt aggregate principal amount	$ 82,057,000	$ 167,943,000	$ 250,000,000
Original Maturity Date			Nov. 14, 2020
Lazard Group 4.50% Senior Notes [Member]
Debt Instrument [Line Items]
Senior debt, initial principal amount			$ 500,000,000	$ 500,000,000
Interest rate, payment terms			Interest on the 2028 Notes is payable semi-annually on March 19 and September 19 of each year, beginning March 19, 2019.
Senior notes interest rate			4.50%	4.50%
Original Maturity Date			Sep. 19, 2028
Effective interest rates of senior notes			4.68%
Lazard Group 3.75% Senior Notes [Member]
Debt Instrument [Line Items]
Senior notes interest rate			3.75%
Original Maturity Date			Feb. 13, 2025
Effective interest rates of senior notes			3.87%
Lazard Group 3.625% Senior Notes [Member]
Debt Instrument [Line Items]
Senior debt, initial principal amount			$ 300,000,000
Senior notes interest rate			3.625%
Original Maturity Date			Mar. 01, 2027
Effective interest rates of senior notes			3.76%