Derivative Financial Instruments (Counterparty Credit Quality With Respect To Fair Value Of OTC Derivatives Assets) (Details) $ in Thousands	May 31, 2019 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
A- or higher	$ 94,830
BBB- to BBB	24,884
BB or lower	194,336
Unrated	131,239
Total OTC derivative assets included in Trading assets	$ 445,289

Fair value of OTC derivatives assets, counterparty credit quality A- or higher.

No definition available.

Name:	jef_FairValueOfOtcDerivativesAssetsCounterpartyCreditQuality
Namespace Prefix:	jef_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of OTC derivatives assets, counterparty credit quality BB or lower.

No definition available.

Name:	jef_FairValueOfOtcDerivativesAssetsCounterpartyCreditQualityBbPlusOrLower
Namespace Prefix:	jef_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of OTC derivatives assets, counterparty credit quality BBB- to BBB+.

No definition available.

Name:	jef_FairValueOfOtcDerivativesAssetsCounterpartyCreditQualityBbbMinusToBbbPlus
Namespace Prefix:	jef_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of OTC derivatives assets, counterparty credit quality unrated.

No definition available.

Name:	jef_FairValueOfOverCounterDerivativesAssetsCounterpartyCreditQualityUnrated
Namespace Prefix:	jef_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

OTC Derivative Assets Included In Financial Instruments Owned.

No definition available.

Name:	jef_OtcDerivativeAssetsIncludedInFinancialInstrumentsOwned
Namespace Prefix:	jef_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

No definition available.

Name:	us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration