Significant Accounting Policies Significant Accounting Policies (Policies)	6 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Consolidation, Policy [Policy Text Block]	Consolidation In the normal course of its business, Legg Mason sponsors and manages various types of investment products. For its services, Legg Mason is entitled to receive management fees and may be eligible, under certain circumstances, to receive additional subordinated management fees or other incentive fees. Legg Mason's exposure to risk in these entities is generally limited to any equity investment it has made or is required to make, and any earned but uncollected management fees. Legg Mason did not sell or transfer investment assets to any of these investment products. In accordance with financial accounting standards, Legg Mason consolidates certain sponsored investment products, some of which are designated and reported as consolidated investment vehicles (“CIVs”). The consolidation of sponsored investment products, including those designated as CIVs, has no impact on Net Income Attributable to Legg Mason, Inc. and does not have a material impact on Legg Mason's consolidated operating results. The change in the value of all consolidated sponsored investment products is recorded in Non-Operating Income (Expense) and reflected in Net income attributable to noncontrolling interests. Certain of the investment products Legg Mason sponsors and manages are considered to be variable interest entities ("VIEs") (as further described below) while others are considered to be voting rights entities (“VREs”) subject to traditional consolidation concepts based on ownership rights. Legg Mason may fund the initial cash investment in certain VRE investment products to generate an investment performance track record in order to attract third-party investors in the product. Legg Mason's initial investment in a new product typically represents 100% of the ownership in that product. As further discussed below, the products with “seed capital investments” are consolidated as long as Legg Mason maintains a controlling financial interest in the product, but they are not designated as CIVs by Legg Mason unless the investment is longer-term. As of September 30, 2017, March 31, 2017, and September 30, 2016, no consolidated VREs were designated as CIVs. A VIE is an entity which does not have adequate equity to finance its activities without additional subordinated financial support; or the equity investors, as a group, do not have the normal characteristics of equity investors for a potential controlling financial interest. Legg Mason must consolidate any VIE for which it is deemed to be the primary beneficiary. Under consolidation accounting guidance, if limited partners or similar equity holders in a sponsored investment vehicle structured as a limited partnership or a similar entity do not have either substantive kick-out or substantive participation rights over the general partner, the entities are VIEs. As a sponsor and manager of an investment vehicle, Legg Mason may be deemed a decision maker under the accounting guidance. If the fees paid to a decision maker are market-based, such fees are not considered variable interests in a VIE. Market-based fees are those fees which are both customary and commensurate with the level of effort required for the services provided. Additionally, if employee interests in a sponsored investment vehicle are not made to circumvent the consolidation guidance and are not financed by the sponsor, they are not included in the variable interests assessment, and are not included in the primary beneficiary determination. A decision maker is deemed to be a primary beneficiary of a VIE if it has the power to direct activities that most significantly impact the economic performance of the VIE and the obligation to absorb losses or receive benefits from variable interests that could be significant to the VIE. In determining whether it is the primary beneficiary of a VIE, Legg Mason considers both qualitative and quantitative factors such as the voting rights of the equity holders, guarantees, and implied relationships. If a fee paid to a decision maker is not market-based, it will be considered in the primary beneficiary determination. Legg Mason concluded it was the primary beneficiary of three foreign mutual fund VIEs as of each September 30, 2017 and March 31, 2017, and of six foreign mutual fund VIEs as of September 30, 2016, which were consolidated and designated as CIVs, because it held significant financial interests in these funds. Legg Mason also concluded it was the primary beneficiary of two sponsored investment fund VIEs, and one employee-owned fund that it sponsors, as of each September 30, 2017, March 31, 2017, and September 30, 2016, which were also consolidated and designated as CIVs. On July 26, 2017, Legg Mason entered into four total return swap arrangements with financial intermediaries with respect to one Legg Mason sponsored exchange traded fund ("ETF") which resulted in investments in the ETF by those intermediaries in the aggregate amount of $23,096. Similarly, on June 6, 2017, Legg Mason entered into four total return swap arrangements with financial intermediaries with respect to another Legg Mason sponsored ETF, which resulted in investments in the ETF by those financial intermediaries in the aggregate amount of $20,253. See Note 12 for additional information regarding the total return swaps. Under the terms of the total return swaps, Legg Mason absorbs all gains and losses on the underlying ETF investments of these financial intermediaries, and therefore has variable interests in each of the two related funds, and is deemed to be the primary beneficiary. As such, each of the two underlying sponsored investment funds were consolidated and designated CIVs as of September 30, 2017. See Notes 4 and 13 for additional information related to VIEs and CIVs.
Commitments and Contingencies, Policy [Policy Text Block]	Contingent Consideration Liabilities In connection with business acquisitions, Legg Mason may be required to pay additional future consideration based on the achievement of certain designated financial metrics. Legg Mason estimates the fair value of these potential future obligations at the time a business combination is consummated and records a Contingent consideration liability in the Consolidated Balance Sheet. Legg Mason accretes contingent consideration liabilities to the expected payment amounts over the related earn-out terms until the obligations are ultimately paid, resulting in Interest expense in the Consolidated Statements of Income. If the expected payment amounts subsequently change, the contingent consideration liabilities are (reduced) or increased in the current period, resulting in a (gain) or loss, which is reflected within Contingent consideration fair value adjustments in the Consolidated Statements of Income. See Notes 3 and 9 for additional information regarding contingent consideration liabilities.
Noncontrolling Interests Redeemable [Policy Text Block]	Noncontrolling Interests Noncontrolling interests include affiliate minority interests, third-party investor equity in consolidated sponsored investment products, and vested affiliate management equity plan interests. For CIVs and other consolidated sponsored investment products with third-party investors, the related noncontrolling interests are classified as redeemable noncontrolling interests if investors in these funds may request withdrawals at any time. Also included in redeemable noncontrolling interests are vested affiliate management equity plan and affiliate minority interests for which the holder may, at some point, request settlement of their interests. Redeemable noncontrolling interests are reported in the Consolidated Balance Sheets at their estimated settlement values. Changes in the expected settlement values are recognized over the settlement period as adjustments to retained earnings. Nonredeemable noncontrolling interests include vested affiliate management equity plan interests that do not permit the holder to request settlement of their interests. Nonredeemable noncontrolling interests are reported in the Consolidated Balance Sheets at their issuance value, together with undistributed net income allocated to noncontrolling interests. Legg Mason estimates the settlement value of noncontrolling interests as their fair value. For consolidated sponsored investment products, where the investor may request withdrawal at any time, fair value is based on market quotes of the underlying securities held by the investment products. For affiliate minority interests and management equity plan interests, fair value reflects the related total business enterprise value, after appropriate discounts for lack of marketability and control. There may also be features of these equity interests, such as dividend subordination, that are contemplated in their valuations. The fair value of option-like management equity plan interests also relies on Black-Scholes option pricing model calculations. Net income attributable to noncontrolling interests in the Consolidated Statements of Income includes the share of net income of the respective subsidiary allocated to the minority interest holders. See Note 11 for additional information regarding noncontrolling interests.
Derivatives, Policy [Policy Text Block]	Derivative Instruments As noted above, on July 26, 2017, Legg Mason entered into four total return swap arrangements with financial intermediaries with respect to one Legg Mason sponsored ETF for an aggregate notional amount of $23,096 which resulted in investments in the ETF by each of those financial intermediaries. Similarly, on June 6, 2017, Legg Mason entered into four total return swap arrangements with financial intermediaries with respect to another Legg Mason sponsored ETF for an aggregate notional amount of $20,253, which resulted in investments in that ETF by each of those financial intermediaries. As of September 30, 2017, the aggregate notional amounts related to these total return swap arrangements totaled $44,464. The total return swap arrangements qualify as derivative instruments and are not designated for hedge accounting. In connection with the arrangements, Legg Mason also executed futures contracts to partially hedge the market risk related to the total return swap arrangements.
Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]	Stock-Based Compensation Effective April 1, 2017, Legg Mason adopted updated accounting guidance on stock-based compensation accounting. The updated guidance simplifies several aspects of accounting for stock-based compensation including the income tax consequences, and clarifies classification criteria for awards as either equity or liabilities, and the classification of related amounts in statements of cash flows. The updated guidance requires all excess tax benefits and deficiencies associated with stock-based compensation to be recognized as discrete items in the Income tax provision in the Consolidated Statements of Income in the reporting period in which they occur, thereby increasing the volatility of the Income tax provision as a result of fluctuations in Legg Mason's stock price. Legg Mason adopted this amendment on a modified retrospective basis, and recorded a cumulative-effect adjustment of $24,327 as an increase to both deferred tax assets and Retained earnings on the Consolidated Balance Sheet as of April 1, 2017. These tax benefits were not previously recognized due to Legg Mason's cumulative tax loss position. In addition, Legg Mason recorded a related discrete Income tax expense of $214 and $1,120 during the three and six months ended September 30, 2017, respectively, for vested stock awards with a grant date exercise price higher than the related vesting date stock price, as this aspect of the guidance was adopted on a prospective basis. Upon adoption of the updated guidance, Legg Mason elected to prospectively account for forfeitures as they occur, which did not have a material impact on the Consolidated Financial Statements. Also, cash flows related to income tax deductions in excess of or less than the related stock-based compensation expense will be classified as Cash Flows from Operating Activities in the Consolidated Statements of Cash Flows.
Comprehensive Income, Policy [Policy Text Block]	Accumulated Other Comprehensive Loss, Net There were no significant amounts reclassified from Accumulated other comprehensive loss, net, to the Consolidated Statements of Income, except as follows. During the three months and six months ended September 30, 2017, $121 and $240, respectively, of previously unrecognized losses on a defined benefit pension plan were reclassified and expensed as further described in Note 3, and during the three and six months ended September 30, 2016, $3,899 and $4,426, respectively, realized on the settlement of an interest rate swap was reclassified and expensed, as further described in Note 7.
Income Tax, Policy [Policy Text Block]	Income Taxes Noncontrolling interests in EnTrustPermal Group Holdings, LLC ("EnTrustPermal"), Clarion Partners, LLC ("Clarion Partners") and Royce & Associates ("Royce") are structured as partnerships that pass an allocable portion of tax attributes and obligations to the related noncontrolling interest holders. As such, the consolidated financial statements do not generally include any tax provision/benefit associated with the net income allocated to these noncontrolling interests, which caused a reduction in the effective tax rate of 2.8 percentage points and 6.7 percentage points for the three months ended September 30, 2017 and 2016, respectively, and 3.6 percentage points and 4.4 percentage points for the six months ended September 30, 2017 and 2016, respectively. In connection with the adoption of updated accounting guidance on stock-based compensation accounting discussed above, Legg Mason recorded a discrete income tax expense of approximately $214 and $1,120 during the three and six months ended September 30, 2017, respectively, which increased the effective tax rate by 0.2 percentage point and 0.5 percentage points, respectively.
New Accounting Pronouncements, Policy [Policy Text Block]	Recent Accounting Developments In August 2017, the Financial Accounting Standards Board ("FASB") updated the guidance on accounting for derivative hedging. The updated guidance more closely aligns the results of cash flow and fair value hedging designations with risk management activities through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results in the financial statements. The new guidance also simplifies the application of hedge accounting. The updated guidance is effective for Legg Mason in fiscal 2020, unless adopted earlier. Legg Mason only uses accounting hedge designation from time-to-time and would only be impacted if derivative transactions were designated for hedging. In February 2016, the FASB updated the guidance on accounting for leases. The updated guidance requires that a lessee shall recognize the assets and liabilities that arise from lease transactions. A lessee will recognize a right-of-use asset to use the underlying asset and a liability representing the lease payments. The updated guidance also requires an evaluation at the inception of a service or other contract, to determine whether the contract is or contains a lease. The guidance will be effective for Legg Mason in fiscal 2020. Legg Mason expects to recognize right of use assets and liabilities upon its adoption of the new standard and is continuing to evaluate the full impact of its adoption. In May 2014, the FASB updated the guidance on revenue recognition. The updated guidance provides a single, comprehensive revenue recognition model for all contracts with customers, improves comparability and removes inconsistencies in revenue recognition practices across entities, industries, jurisdictions, and capital markets. The guidance also requires comprehensive disclosures about the nature, timing, and uncertainty of revenue and cash flows arising from contracts with customers, including significant judgments made in applying the guidance. In March 2016, the FASB further updated the revenue guidance on determining whether to report revenue on a gross versus net basis. The updated guidance clarifies how entities evaluate principal versus agent aspects of the revenue recognition guidance issued in May 2014. The evaluation will require entities to identify all goods or services to be provided to the customer, and determine whether they obtain control of the good or service before it is transferred to the customer, where control would suggest a principal relationship, which would be accounted for on a gross basis. Legg Mason has reviewed its revenue contracts, and is monitoring relevant implementation guidance. Legg Mason does not anticipate any significant changes to current revenue recognition practices, except as discussed below. Legg Mason may be required to recognize longer-term performance and incentive fees subject to clawback when clawback is not reasonably possible. This is earlier than under its current revenue recognition process, which defers recognition until all contingencies are resolved. Additionally, Legg Mason is evaluating whether certain separate account commissions currently expensed when paid meet the criteria for capitalization and amortization. Legg Mason is also evaluating whether revenue-related costs currently presented on a gross basis will be recorded net, or vice versa. The evaluation of the effect of this guidance is ongoing, and Legg Mason has not determined the ultimate impact of the adoption or the transition method to be used upon adoption, which is effective for Legg Mason on April 1, 2018.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.