Income Taxes Valuation Allowances (Details) - USD ($) $ in Thousands		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2017
Valuation Allowance [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards		$ 3,121	$ 6,183
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		258,486	283,746
Deferred Tax Assets, Charitable Contribution Carryforwards		4,552	814
Deferred Tax Assets, Tax Credit Carryforwards		539,781	537,723
Deferred Tax Assets, Valuation Allowance		79,476	71,063
Operating Loss Carryforwards, Valuation Allowance		71,518	57,406
Domestic Tax Authority [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	[1]	82,350	57,715
Deferred Tax Assets, Capital Loss Carryforwards		0	3,644
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		258,486	283,746
Deferred Tax Assets, Charitable Contribution Carryforwards		4,552	814
Deferred Tax Assets, Valuation Allowance			11,133
Domestic Tax Authority [Member] | Valuation Allowance, Operating Loss Carryforwards [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		2,255	2,856
Domestic Tax Authority [Member] | Valuation Allowance, Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		15,252	6,147
Domestic Tax Authority [Member] | Contribution of Monetary Assets by an Enterprise to a Charitable Organization [Domain]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		3,443	814
NEW YORK
Valuation Allowance [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results		17,053
State and Local Jurisdiction [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards		44	4
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	[2],[3]	166,772	168,655
Deferred Tax Assets, Tax Credit Carryforwards		308	366
State and Local Jurisdiction [Member] | Valuation Allowance, Operating Loss Carryforwards [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		26,816	29,075
State and Local Jurisdiction [Member] | Capital Loss Carryforward [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		44	4
Foreign Tax Authority [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards		3,077	2,535
Deferred Tax Assets, Operating Loss Carryforwards, Foreign		24,192	20,244
Foreign Tax Authority [Member] | Valuation Allowance, Operating Loss Carryforwards [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		20,631	15,975
Foreign Tax Authority [Member] | Capital Loss Carryforward [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		3,077	2,535
Foreign Tax Authority [Member] | Valuation Allowance, Other Tax Carryforward [Member]
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance		$ 7,958	$ 13,657

[1] Currently, there are proposals to significantly change U.S. federal income tax rules, which include proposals to reduce tax rates, limit deductibility of interest expense, and changes to the taxation on non-U.S. earnings. Although details are not yet available, these changes could impact the carrying value of Legg Mason's deferred tax assets.

[2] Due to potential for change in the factors relating to apportionment of income to various states, Legg Mason's effective state tax rates are subject to fluctuation which will impact the value of the Company's deferred tax assets, including net operating losses, and could have a material impact on the future effective tax rate of the Company.

[3] Substantially all of the U.S. state net operating losses carryforward through fiscal 2036.