Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Feb. 28, 2014	Aug. 31, 2015
Commitments Contingencies And Guarantees [Line Items]
Loan commitments outstanding to clients		$ 613.6
Fair value of mortgage-related commitments		187.7
Litigation settlement amount	$ 25.0
Litigation reserve		0.6
Fair value of derivative contracts approximated deemed to meet the definition of a guarantee		620.0
Maximum amount payable under guarantee		28.8
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Letters of credit commitments		39.5
Jefferies Capital Partners LLC [Member]
Commitments Contingencies And Guarantees [Line Items]
Outstanding equity commitments		25.4
Other Investments [Member]
Commitments Contingencies And Guarantees [Line Items]
Outstanding equity commitments		$ 4.4