Income Taxes Deferred Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Deferred Income Taxes [Line Items]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	$ 182,734	$ 149,891
Deferred Tax Assets, Tax Deferred Expense, Other	39,961	35,577
Deferred Tax Assets, Goodwill and Intangible Assets	23,480	0
Deferred Tax Assets, Operating Loss Carryforwards	325,282	352,300
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	266,128	278,131
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Loss Reserves	6,597	7,147
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Other	12,926	14,619
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions	8,694	5,909
Deferred Tax Assets, Charitable Contribution Carryforwards	0	973
Deferred Tax Assets, Other	2,155	7,643
Deferred Tax Assets, Gross	867,957	852,190
Deferred Tax Assets, Valuation Allowance	(134,209)	(126,538)
Deferred Tax Assets, Net of Valuation Allowance	733,748	725,652
Deferred Tax Assets, Net	76,297	62,281
Deferred Tax Liabilities, Property, Plant and Equipment	636,230	588,890
Deferred Tax Liabilities, Goodwill and Intangible Assets	0	2,262
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	2,007	5,796
Deferred Tax Liabilities, Gross	657,451	663,371
Deferred Tax Liabilities, Other	19,214	66,423
U.S. Earnings	3,662,000
Foreign Earnings	331,000
EnTrust Global [Member]
Deferred Income Taxes [Line Items]
Deferred Tax Assets, Other	53,764
Deferred Tax Liabilities, Other		1,510	$ 4,370
Increase (Decrease) in Deferred Income Taxes			4,102
UNITED STATES
Deferred Income Taxes [Line Items]
Deferred Tax Assets, Gross	594,583
State and Local Jurisdiction [Member]
Deferred Income Taxes [Line Items]
Deferred Tax Assets, Gross	225,727
Deferred Tax Assets, Valuation Allowance	(93,185)
SEC Schedule, 12-09, Valuation Allowance, Tax Credit Carryforward [Member]
Deferred Income Taxes [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	1,800,000
SEC Schedule, 12-09, Valuation Allowance, Other Tax Carryforward [Member]
Deferred Income Taxes [Line Items]
Deferred Tax Assets, Valuation Allowance	(25,405)	(17,411)
SEC Schedule, 12-09, Valuation Allowance, Other Tax Carryforward [Member] | UNITED KINGDOM
Deferred Income Taxes [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	6,144
Consolidated Legg Mason, Inc. | RETAINED EARNINGS
Deferred Income Taxes [Line Items]
Excess Tax Benefit from Share-based Compensation, Financing Activities	$ 0	$ 24,327	$ 0