Significant Accounting Policies Significant Accounting Policies (Policies)	9 Months Ended
	Dec. 31, 2015
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 vehicles. 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 assets to any of these investment vehicles. In accordance with financial accounting standards, Legg Mason consolidates certain sponsored investment vehicles, some of which are designated as consolidated investment vehicles (“CIVs”). The consolidation of sponsored investment vehicles, including those designated as CIVs, has no impact on Net Income (Loss) 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 vehicles, which is recorded in Other Non-Operating Income (Expense), is reflected in Net Income (Loss), net of amounts allocated to noncontrolling interests. Certain investment vehicles Legg Mason sponsors and is the manager of 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. Sponsored investment vehicles that are considered VREs are consolidated if Legg Mason has a controlling financial interest in the investment vehicle, absent substantive investor rights to replace the manager of the entity (kick-out rights). Legg Mason may also fund the initial cash investment in certain VRE investment vehicles 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, these “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. Legg Mason held a longer-term controlling financial interest in one sponsored investment fund VRE, which has third-party investors and was consolidated and included as a CIV prior to the quarter ended March 31, 2015. Prior to March 31, 2015, Legg Mason redeemed a significant portion of its investment in this fund and as a result no longer had a controlling financial interest in the fund; therefore, the fund was not included as a CIV as of or subsequent to March 31, 2015. 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.
Consolidation, Variable Interest Entity, Policy [Policy Text Block]	Investment Company VIEs For most sponsored investment fund VIEs deemed to be investment companies, including money market funds, Legg Mason determines it is the primary beneficiary of the VIE if it absorbs a majority of the VIE's expected losses, or receives a majority of the VIE's expected residual returns, if any. Legg Mason's determination of expected residual returns excludes gross fees paid to a decision maker if certain criteria relating to the fees are met. In determining whether it is the primary beneficiary of an investment company VIE, Legg Mason considers both qualitative and quantitative factors such as the voting rights of the equity holders; economic participation of all parties, including how fees are earned and paid to Legg Mason; related party (including employees) ownership; guarantees and implied relationships. Legg Mason concluded it was the primary beneficiary of one sponsored investment fund VIE, which was consolidated (and designated as a CIV) as of December 31, 2015, March 31, 2015, and December 31, 2014, despite significant third-party investments in this product. As of December 31, 2015, March 31, 2015, and December 31, 2014, Legg Mason also concluded it was the primary beneficiary of 14, 17, and 16 employee-owned funds it sponsors, respectively, which were consolidated and reported as CIVs. Other VIEs For other sponsored investment funds that do not meet the investment company criteria, Legg Mason determines it is the primary beneficiary of a VIE if it has both the power to direct the activities of the VIE that most significantly impact the entity's economic performance and the obligation to absorb losses, or the right to receive benefits, that potentially could be significant to the VIE. As of December 31, 2015, Legg Mason had a variable interest in three collateralized loan obligations ("CLOs"). Legg Mason concluded it was not the primary beneficiary of these CLOs, which were not consolidated, as it holds no equity interest in these investment vehicles and the level of fees they are expected to pay to Legg Mason is insignificant. Prior to June 30, 2014, Legg Mason concluded that it was the primary beneficiary of another CLO in which it held a variable interest and balances related to this CLO were consolidated and reported as a CIV in the Company's consolidated financial statements. Although it held no equity interest in this investment vehicle, it had both the power to control the CLO and had a significant variable interest because of the level of its expected subordinated fees. During the three months ended June 30, 2014, this CLO was substantially liquidated and therefore was not consolidated by Legg Mason as of, or subsequent to, June 30, 2014. See Notes 4 and 13 for additional information regarding VIEs and VREs.
Contingent Liability Reserve Estimate, 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 (Loss). 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 Other operating expense in the Consolidated Statements of Income (Loss).
Noncontrolling Interests Redeemable [Policy Text Block]	Noncontrolling Interests Noncontrolling interests include affiliate minority interests, third-party investor equity in consolidated investment vehicles, and vested management equity plan interests. For CIVs and other consolidated sponsored investment vehicles 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 interests, including accretion of related estimated redemption values. There were no nonredeemable noncontrolling interests as of December 31, 2015 or March 31, 2015. See Note 11 for additional information regarding noncontrolling interests.
Comprehensive Income, Policy [Policy Text Block]	Accumulated Other Comprehensive Income (Loss), Net There were no significant amounts reclassified from Accumulated other comprehensive income (loss), net, to the Consolidated Statements of Income (Loss) for the nine months ended December 31, 2015 or 2014, except for $638 realized on the termination of a reverse treasury rate lock contract in July 2014, as further described in Note 7.
Income Tax, Policy [Policy Text Block]	Income Tax Provision (Benefit) During the three months ended December 31, 2015, Legg Mason recognized a cumulative income tax benefit of $55,842, primarily related to annualized tax benefits attributable to prior quarters due to $371,000 of non-cash impairment charges recognized in the current quarter in a lower tax rate jurisdiction. See Note 6 for additional information regarding the impairment charges. In November 2015, the U.K. Finance Bill 2015 was enacted, which reduced the main U.K. corporate tax rate from 20% to 19% effective April 1, 2017, and to 18% effective April 1, 2020. The reduction in the U.K. corporate tax rate resulted in a tax benefit of $8,361, recognized in the three months ended December 31, 2015, as a result of the revaluation of certain existing deferred tax assets and liabilities at the new rates. Also, in the three months ended December 31, 2015, Legg Mason recognized income tax benefits of $7,216 which resulted from reserve adjustments related to the effective settlement of tax positions in certain tax examinations. These benefits were offset in part by an increase in valuation allowances of $8,479 related to foreign tax credits, charitable contributions and certain state net operating loss carryforwards. In addition, during the three months ended September 30, 2015, Legg Mason recognized income tax benefits of $7,026 as a result of reserve adjustments related to the conclusion of certain tax examinations, and during the three months ended June 30, 2015, Legg Mason recognized an income tax benefit of $17,527 as a result of an increase in the value of deferred tax assets due to changes in the New York City tax code. During the three months ended December 31, 2014, the net release of reserves for uncertain tax positions, due to audit settlements during the period, resulted in a tax benefit of approximately $4,300. The net release of reserves reduced the effective tax rate by 3.7 percentage points and 1.8 percentage points, for the three and nine months ended December 31, 2014, respectively.
New Accounting Pronouncements, Policy [Policy Text Block]	Recent Accounting Developments In November 2015, the Financial Accounting Standards Board (“FASB”) updated the guidance on balance sheet classification of deferred taxes. The updated guidance requires that all deferred tax liabilities and assets be classified as noncurrent in a classified balance sheet. The guidance will be effective for Legg Mason in fiscal 2017, unless adopted earlier. Legg Mason is evaluating the impact of its adoption. In May 2015, the FASB updated the guidance on fair value measurement. The updated guidance removes the requirement for all investments for which fair value is measured using the net asset value ("NAV") practical expedient to be categorized within the fair value hierarchy and related sensitivity disclosures. The amount of such investments would instead be disclosed as a reconciling item between the fair value hierarchy table and the investment amounts reported on the balance sheet. This guidance will be effective for Legg Mason in fiscal 2017, unless adopted earlier. Legg Mason is evaluating the impact of its adoption. In February 2015, the FASB updated the guidance for consolidation requirements. The updated guidance eliminates the presumption that a general partner should consolidate a limited partnership, and modifies the evaluation of whether limited partnerships and similar legal entities are VIEs or VREs. Additionally, the updated guidance affects the conclusion such that certain fees paid to decision makers are no longer variable interests, and certain related party relationships with a sponsored investment fund may no longer require its consolidation. The update also eliminates the deferral of accounting guidance that requires separate evaluation for investment company VIEs and other VIEs. This update will be effective for Legg Mason in fiscal 2017, unless adopted earlier. Legg Mason is evaluating the timing and impact of its adoption. In May 2014, the FASB updated the guidance on revenue recognition. The updated guidance improves comparability and removes inconsistencies in revenue recognition practices across entities, industries, jurisdictions, and capital markets. This update has been deferred for an additional year and will now be effective for Legg Mason in fiscal 2019. Legg Mason is evaluating the impact of its adoption.

Disclosure of accounting policy for consolidation to describe the significant judgments and assumptions made in determining whether a variable interest held by the entity requires the variable interest entity to be consolidated and (or) disclose information about its involvement with the variable interest entity; the methodology used by the entity for determining whether or not it is the primary beneficiary of the variable interest entity; and the significant factors considered and judgments made in determining that the power to direct the activities that significantly impact the economic performance of the variable interest entity are shared (as defined).

- Definition

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.

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