Commitments, Contingencies and Guarantees - Additional Information (Detail) £ in Millions, $ in Millions	1 Months Ended	9 Months Ended
	Mar. 31, 2016 GBP (£)	Aug. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Contractual obligation | £	£ 8.1
Contractual obligation period for 18 years	18 years
Loan commitments outstanding to clients		$ 192.9
Fair value of mortgage-related commitments		185.2
Fair value of derivative contracts approximated deemed to meet the definition of a guarantee		320.3
Maximum amount payable under guarantee		21.3
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit commitments		$ 36.5
Standby letters of credit expiration period		2 years
Jefferies Capital Partners LLC [Member]
Loss Contingencies [Line Items]
Outstanding equity commitments		$ 23.1
Other Investments [Member]
Loss Contingencies [Line Items]
Outstanding equity commitments		$ 4.3