Acquisitions - Summary of Fair Value of Assets Acquired and Liabilities Assumed at Acquisition Date (Detail) - USD ($) $ in Thousands			12 Months Ended
	Sep. 01, 2015	Aug. 04, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Purchase Price:
Cash paid			$ 2,166	$ 1,000	$ 0
Fair Value of Liabilities Assumed:
Purchase price allocated to goodwill			3,829	$ 2,570	$ 0
MLV [Member]
Purchase Price:
Cash paid	$ 3,250
Total purchase price	3,250
Fair Value of Assets Acquired:
Cash	975
Due from brokers, dealers and clearing organizations	2,098
Investment banking receivables and Securities borrowed	1,111
Other	606
Total	4,790
Fair Value of Liabilities Assumed:
Accounts payable, accrued expenses and other liabilities	3,305
Fair value of net assets acquired	1,485
Purchase price allocated to goodwill	1,259
Purchase price allocated to intangible assets	506
Total purchase price	$ 3,250
Lazard Capital Markets LLC [Member]
Purchase Price:
Cash paid		$ 1,000	$ 2,166
Total purchase price		4,070
Total		5,070
Fair Value of Assets Acquired:
Due from brokers, dealers and clearing organizations		357
Investment banking receivables and Securities borrowed		675,709
Total		676,066
Fair Value of Liabilities Assumed:
Securities loaned		676,066
Fair value of net assets acquired		5,070
Purchase price allocated to goodwill		2,570
Purchase price allocated to intangible assets		2,500
Total purchase price		$ 4,070