Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended
	Jan. 31, 2013	May. 31, 2015
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients		$ 607.5
Fair value of mortgage-related commitments		135.7
Litigation settlement amount	$ 25.0
Litigation reserve		0.7
Fair value of derivative contracts approximated deemed to meet the definition of a guarantee		371.6
Maximum amount payable under guarantee		29.1
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit commitments		37.3
Jefferies Capital Partners LLC [Member]
Commitments Contingencies And Guarantees [Line Items]
Outstanding equity commitments		30.2
Other Investments [Member]
Commitments Contingencies And Guarantees [Line Items]
Outstanding equity commitments		$ 4.4