Basis of Presentation and Significant Accounting Policies (Policy)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Principal Transactions Revenues	Principal Transactions Revenues Trading assets and trading liabilities (all of which are recorded on a trade-date basis) are carried at fair value with gains and losses reflected in Principal transaction revenues in our Consolidated Statements of Operations, except for derivatives accounted for as hedges (see “Hedge Accounting” section herein and Note 4). Fees received on loans carried at fair value are also recorded within Principal transaction revenues.
Hedge Accounting	Hedge Accounting Jefferies applies hedge accounting using interest rate swaps designated as fair value hedges of changes in the benchmark interest rate of fixed rate senior long-term debt. Jefferies interest rate swaps are included as derivative contracts in Trading assets and Trading liabilities in the Consolidated Statements of Financial Condition. Jefferies uses regression analysis to perform ongoing prospective and retrospective assessments of the effectiveness of these hedging relationships. A hedging relationship is deemed effective if the change in fair value of the interest rate swap and the change in the fair value of the long-term debt due to changes in the benchmark interest rate offset within a range of 80% to 125%. The impact of valuation adjustments related to Jefferies own credit spreads and counterparty credit spreads are included in the assessment of effectiveness. For qualifying fair value hedges of benchmark interest rates, the change in the fair value of the derivative and the change in fair value of the long-term debt provide offset of one another, and together with any resulting ineffectiveness, are recorded in Interest expense. See Note 4 for further information.
Receivables	Receivables At September 30, 2017 and December 31, 2016, Receivables include receivables from brokers, dealers and clearing organizations of $2,674.7 million and $2,062.9 million, respectively, and receivables from customers of securities operations of $1,291.5 million and $843.1 million, respectively.
Payables, expense accruals and other liabilities	Payables, expense accruals and other liabilities At September 30, 2017 and December 31, 2016, Payables, expense accruals and other liabilities include payables to brokers, dealers and clearing organizations of $2,668.9 million and $3,290.4 million, respectively, and payables to customers of securities operations of $2,459.7 million and $2,297.3 million, respectively.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets We evaluate our long-lived assets for impairment whenever events or changes in circumstances indicate, in management's judgment that the carrying value of such assets may not be recoverable. In the third quarter of 2016, JETX recorded impairment charges in Selling, general and other expenses of $55.0 million related to write-downs of unproved oil and gas properties. JETX assesses its unproved oil and gas properties for impairment based on remaining lease terms, drilling results or future plans to develop acreage and they record impairment expense for any decline in value. In the third quarter of 2016, JETX curtailed development of its southern acreage in the East Eagle Ford and its Houston County acreage. As a result, an impairment was recorded for the difference between the carrying value and the estimated net realizable value of the acreage. In the third quarter of 2017, JETX decided to focus on its other higher returning acreage and as a result wrote off the remaining carrying value for its Houston County acreage. JETX recorded an impairment charge of $10.1 million in Selling, general and other expenses in the third quarter of 2017 related to the write-down of unproved oil and gas properties.
Accounting Developments	Accounting Developments - Accounting Standards to be Adopted in Future Periods Revenue Recognition. In May 2014, the FASB issued new guidance that defines how companies report revenues from contracts with customers, and also requires enhanced disclosures. The core principle of this new guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. This guidance is effective for interim and annual periods beginning after December 15, 2017. We intend to adopt the new guidance using the modified retrospective approach with a cumulative-effect adjustment to opening retained earnings. The evaluation of the impact this new guidance will have on our consolidated financial statements is ongoing. Our implementation efforts include the identification of revenue streams within the scope of the guidance, the evaluation of certain revenue contracts and education and discussion with our consolidated subsidiaries. The below focuses on our two largest consolidated subsidiaries, whose combined revenues make up more than 92% of our Net revenues in the Consolidated Statements of Operations for the nine months ended September 30, 2017. We continue to evaluate revenues for our consolidated subsidiaries not disclosed below, however, we do not expect the adoption of the new guidance for these subsidiaries to have a material impact on our consolidated financial statements. For Jefferies, as the new revenue recognition guidance does not apply to revenue associated with financial instruments, including loans and securities that are accounted for under other GAAP, Jefferies does not expect the guidance to have a material impact on the elements of its revenues most closely associated with financial instruments, including Principal transaction revenues, Interest income and Interest expense. Jefferies evaluation of the impact of the new guidance on its consolidated financial statements is ongoing. Jefferies continues to evaluate the timing of recognition for each revenue stream within scope, which includes investment banking and asset management fees which may be accelerated or deferred depending on the features of the client arrangements, and the timing and presentation of certain related investment banking expenses (whether presented gross or offset against revenues). National Beef's evaluation of the impact of the new guidance on its consolidated financial statements is ongoing. National Beef continues to evaluate its revenue streams and its contracts with customers within each revenue stream, however, National Beef does not expect the adoption of the new revenue standard to materially impact its consolidated financial statements. Financial Instruments. In January 2016, the FASB issued new guidance that affects the accounting for equity investments, financial liabilities under the fair value option and the presentation and disclosure requirements for financial instruments. The guidance is effective for annual and interim periods beginning after December 15, 2017. We are currently evaluating the impact of the new guidance related to equity investments and the presentation and disclosure requirements of financial instruments on our consolidated financial statements. Early adoption was permitted for the accounting guidance on financial liabilities under the fair value option and we adopted this guidance in the first quarter of 2016. The adoption of the guidance on financial liabilities under the fair value option did not have a significant impact on our consolidated financial statements. Leases. In February 2016, the FASB issued new guidance that affects the accounting and disclosure requirements for leases. The FASB requires the recognition of lease assets and lease liabilities on the statement of financial condition. The guidance is effective for annual and interim periods beginning after December 15, 2018. We are currently evaluating the impact this new guidance will have on our consolidated financial statements. Financial Instruments - Credit Losses. In June 2016, the FASB issued new guidance for estimating credit losses on certain types of financial instruments by introducing an approach based on expected losses. The guidance is effective for annual and interim periods beginning after December 15, 2019. We are currently evaluating the impact this new guidance will have on our consolidated financial statements. Cash Flow Classifications. In August 2016, the FASB issued new guidance to reduce the diversity in practice in how certain transactions are classified in the statement of cash flows. The guidance adds or clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows. The guidance is effective for annual and interim periods beginning after December 15, 2017. In November 2016, the FASB issued new guidance on restricted cash. The guidance requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. The guidance is effective for annual and interim periods beginning after December 15, 2017. We are currently evaluating the impact this new guidance will have on our consolidated financial statements. Goodwill. In January 2017, the FASB issued new guidance for simplifying goodwill impairment testing. The guidance is effective for annual and interim periods beginning after December 15, 2019 and early adoption is permitted. We are currently evaluating the impact this new guidance will have on our consolidated financial statements. Retirement Benefits. In March 2017, the FASB issued new guidance for improving the presentation of net periodic pension costs in the statement of operations. The update also allows the service cost to be eligible for capitalization, when applicable. The guidance is effective for annual and interim periods beginning after December 15, 2017 and early adoption is permitted. We are currently evaluating the impact of the new guidance on our consolidated financial statements. Compensation. In May 2017, the FASB issued new guidance providing clarity and reducing diversity in practice and cost and complexity when accounting for a change to the terms or conditions of a share-based payment award. The guidance is effective for annual and interim periods beginning after December 15, 2017 and early adoption is permitted. We are currently evaluating the impact of the new guidance on our consolidated financial statements. Derivatives and hedging. In August 2017, the FASB issued new guidance to improve the financial reporting of hedging relationships to better portray the economic results of an entity's risk management activities in its financial statements. The guidance is effective for annual and interim periods beginning after December 15, 2018 and early adoption is permitted. We are currently evaluating the impact of the new guidance on our consolidated financial statements. Accounting Developments - Adopted Accounting Standards Share-Based Payments to Employees. In January 2017, we adopted the FASB's new guidance that simplifies and improves accounting for share-based payments. The amendments include the recognition of all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations and changes to the timing of recognition of excess tax benefits, the accounting for forfeitures, classification of awards as either equity or liabilities and classification on the statement of cash flows. The adoption of this guidance did not have a significant impact on our consolidated financial statements. We elected to account for forfeitures as they occur, which results in dividends and dividend equivalents originally charged against retained earnings for forfeited shares to be reclassified to compensation expense in the period in which the forfeiture occurs.