Significant Accounting Policies Significant Accounting Policies (Policies)	9 Months Ended
	Dec. 31, 2016
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 (loss) 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. Sponsored investment products 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 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, 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. As of December 31, 2016, March 31, 2016, and December 31, 2015, 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. Updated Consolidation Accounting Guidance Effective April 1, 2016, Legg Mason adopted updated consolidation accounting guidance on a modified retrospective basis. Under the updated 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. 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. The adoption of this accounting guidance as of April 1, 2016, resulted in certain sponsored investment products that reside in foreign mutual fund trusts that were previously accounted for as VREs to be evaluated as VIEs, and the consolidation of nine funds, which were also designated as CIVs. Under the updated accounting guidance, Legg Mason also concluded it was the primary beneficiary of one EnTrust Capital ("EnTrust") sponsored investment fund VIE, which was consolidated and designated a CIV upon the merger of EnTrust and The Permal Group, Ltd. ("Permal"). The adoption also resulted in the deconsolidation of 13 of 14 previously consolidated employee-owned funds, as Legg Mason no longer has a variable interest in those 13 funds. As of December 31, 2016, Legg Mason no longer held a significant financial interest in six of the above foreign mutual funds, and therefore concluded it was no longer the primary beneficiary. As a result, these six funds were not consolidated as of December 31, 2016. In addition, during the quarter ended December 31, 2016, Legg Mason concluded that it was the primary beneficiary of one additional foreign mutual fund, which was consolidated and designated as a CIV. Legg Mason also concluded it was the primary beneficiary of one sponsored investment fund VIE, which was consolidated (and designated as a CIV) as of December 31, 2016. This sponsored investment fund was also consolidated under prior accounting guidance, as further discussed below. The impact of the adoption of the updated accounting guidance on the Consolidated Balance Sheet as of December 31, 2016 was the addition of $16,025 of assets, $2,680 of liabilities, and $18,818 of redeemable noncontrolling interests from CIVs. Prior Consolidation Accounting Guidance Under prior accounting guidance, for most sponsored investment fund VIEs deemed to be investment companies, including money market funds, Legg Mason determined it was the primary beneficiary of a VIE if it absorbed a majority of the VIE's expected losses, or received a majority of the VIE's expected residual returns, if any. Legg Mason's determination of expected residual returns excluded gross fees paid to a decision maker if certain criteria relating to the fees were met. In determining whether it was the primary beneficiary of a VIE, Legg Mason considered both qualitative and quantitative factors such as the voting rights of the equity holders, economic participation of all parties (including how fees were earned and paid to Legg Mason), related party ownership, guarantees, and implied relationships. For other sponsored investment funds that did not meet the investment company criteria, Legg Mason determined it was the primary beneficiary of a VIE if it had both the power to direct the activities of the VIE that most significantly impacted the entity's economic performance and the obligation to absorb losses, or the right to receive benefits, that could have been significant to the VIE. Legg Mason concluded it was the primary beneficiary of one sponsored investment fund VIE, that was consolidated as of March 31, 2016, and December 31, 2015, despite significant third-party investments in this product. Also, as of both March 31, 2016, and December 31, 2015, Legg Mason concluded it was the primary beneficiary of 14 employee-owned funds it sponsored which were consolidated and designated as CIVs. As discussed above, effective April 1, 2016, under new accounting guidance, all but one of those employee-owned funds no longer qualified as VIEs, and 13 of those employee-owned funds were therefore deconsolidated. 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 held no equity interest in these investment products and the level of fees they were expected to pay to Legg Mason was insignificant. Under the new guidance, effective April 1, 2016, these CLOs no longer qualify as VIEs in which Legg Mason might be a primary beneficiary. See Notes 4 and 13 for additional information related to VIEs.
Fair Value Measurement, Policy [Policy Text Block]	Fair Value Measurements Legg Mason's financial instruments are measured and reported at fair value and are classified and disclosed in one of the following categories (the "fair value hierarchy"): Level 1 — Financial instruments for which prices are quoted in active markets, which, for Legg Mason, include investments in publicly traded mutual funds with quoted market prices and equities listed in active markets and certain derivative instruments. Level 2 — Financial instruments for which prices are quoted for similar assets and liabilities in active markets, prices are quoted for identical or similar assets in inactive markets, or prices are based on observable inputs, other than quoted prices, such as models or other valuation methodologies. Level 3 — Financial instruments for which values are based on unobservable inputs, including those for which there is little or no market activity. As a result of the acquisition of Clarion Partners, LLC ("Clarion Partners") in April 2016, Legg Mason holds investments in real estate fund partnerships and limited liability companies, which are classified as Level 3. The fair values of investments in real estate funds are prepared giving consideration to the income, cost and sales comparison approaches of estimating property value. The income approach estimates an income stream for a property and discounts this income plus a reversion (presumed sale) into a present value at a risk adjusted rate. Yield rates and growth assumptions utilized in this approach are derived from market transactions as well as other financial and industry data. The discount rate and the exit capitalization rate are significant inputs to these valuations. These rates are based on the location, type and nature of each property, and current and anticipated market conditions. The cost approach estimates the replacement cost of the building less physical depreciation plus the land value. The sales comparison approach compares recent transactions to the appraised property. Adjustments are made for dissimilarities which typically provide a range of value. Many factors are also considered in the determination of fair value including, but not limited to, the operating cash flows and financial performance of the properties, property types and geographic locations, the physical condition of the asset, prevailing market capitalization rates, prevailing market discount rates, general economic conditions, economic conditions specific to the market in which the assets are located, and any specific rights or terms associated with the investment. Because of the inherent uncertainties of valuation, the values may materially differ from the values that would be determined by negotiations held between parties in a sale transaction. See Note 4 for additional information regarding fair value measurements.
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 Sheets. 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). 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 (loss) attributable to noncontrolling interests in the Consolidated Statements of Income (Loss) 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.
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 (Loss) for the three and nine months ended December 31, 2016 or 2015, except for $2,493 of cumulative foreign currency translation related to the sale of Legg Mason Poland for the three and nine months ended December 31, 2016, and $4,426 realized on the settlement and termination of an interest rate swap for the nine months ended December 31, 2016, as further described in Note 7.
Income Tax, Policy [Policy Text Block]	Income Tax Provision Noncontrolling interests in EnTrustPermal Group Holdings, LLC ("EnTrustPermal"), Clarion Partners, and Royce & Associates ("Royce") are structured as partnerships that pass related tax attributes 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 the effective tax rate to be reduced by 6.2 percentage points and 2.2 percentage points for the three and nine months ended December 31, 2016, respectively. During the three months ended December 31, 2016, an increase in the valuation allowance related to certain state net operating loss carryforwards and foreign tax credits resulted in additional tax expense of $4,755, and increased the effective tax rate by 5.2 percentage points and 1.8 percentage points for the three and nine months ended December 31, 2016, respectively. This expense was offset in part by an income tax benefit of $2,865 recognized during the three months ended December 31, 2016, for provision to return adjustments recognized in connection with the filing of fiscal year 2016 tax returns, which reduced the effective tax rate by 3.1 percentage points and 1.1 percentage points for the three and nine months ended December 31, 2016, respectively. In September 2016, the U.K. Finance Act 2016 was enacted, which reduced the main U.K. corporate tax rate effective on April 1, 2020 from 18% to 17%. The reduction in the U.K. corporate tax rate resulted in a tax benefit of $4,055, recognized in the three months ended September 30, 2016, as a result of the revaluation of certain existing deferred tax assets and liabilities at the new rate, which reduced the effective tax rate by 1.5 percentage points for the nine months ended December 31, 2016. During the three months ended September 30, 2016, Legg Mason also recognized income tax benefits of $2,200 as a result of reserve adjustments related to the conclusion of certain tax examinations, which reduced the effective tax rate by 0.8 percentage points for the nine months ended December 31, 2016. 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 December 2015 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.
New Accounting Pronouncements, Policy [Policy Text Block]	Recent Accounting Developments In January 2017, the Financial Accounting Standards Board ("FASB") updated the guidance to simplify the test for goodwill impairment. The updated guidance still requires entities to perform annual goodwill impairment tests by comparing the fair value of a reporting unit with its related carrying amount, but it eliminates the requirement to potentially calculate the implied fair value of goodwill to determine the amount of impairment, if any. Under the new guidance, an entity should recognize an impairment charge if the reporting unit's carrying amount exceeds the reporting unit’s fair value, in the amount of such excess. The guidance will be effective in fiscal 2020, with the option for early adoption in fiscal 2018. Legg Mason is evaluating its adoption. In August and November 2016, the Financial Accounting Standards Board ("FASB") updated the guidance on the classification of certain cash receipts, cash payments and restricted cash in the statement of cash flows. The updated guidance addresses the reporting classification of several specific cash flow items, including restricted cash, debt prepayment or extinguishment costs, contingent consideration payments, and distributions received from equity method investees, with the objective of reducing diversity in practice where no specific guidance exists, or current guidance is unclear. The updated guidance will be effective in fiscal 2019, with the option for early adoption. Legg Mason is currently evaluating the impact of its adoption. 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 in fiscal 2020. Legg Mason is evaluating the 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. 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. These updates are effective for Legg Mason in fiscal 2019. Legg Mason is evaluating the impact of its adoption.

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.