Consolidated Statements of Cash Flows - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017	Mar. 31, 2016
CASH FLOWS FROM FINANCING ACTIVITIES
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders				$ (23,013)	$ (10,338)
Proceeds from (Payments to) Noncontrolling Interests				19,624	(39,469)
Consolidated Legg Mason, Inc.
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income		$ 87,624	$ 86,532	151,161	131,872
Adjustments to reconcile Net Income to net cash used in operations:
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)				34,000	0
Depreciation and amortization				36,634	40,342
Accretion and amortization of securities discounts and premiums, net				1,670	2,097
Stock-based compensation				39,531	54,981
Deferred income taxes				51,194	30,307
Contingent consideration fair value adjustments		0	(7,000)	(16,550)	(25,000)
Other				(272)	500
Decrease (increase) in assets:
Investment advisory and related fees receivable				(4,978)	(17,298)
Net (purchases) sales of trading and other current investments				32,520	25,002
Other receivables				9,207	(4,030)
Increase (decrease) in liabilities:
Accrued compensation				(96,720)	(102,908)
Deferred compensation				11,386	22,092
Accounts payable and accrued expenses				(9,301)	(3,423)
CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES				175,749	137,859
CASH FLOWS FROM INVESTING ACTIVITIES
Payments for fixed assets				(15,846)	(18,684)
Business acquisitions, net of cash acquired of $29,830				(2,250)	(1,009,928)
Proceeds from Sales of Assets, Investing Activities				2,561	0
Change in restricted cash				(5,103)	436
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES				(15,132)	(1,025,740)
CASH FLOWS FROM FINANCING ACTIVITIES
Business Acquisition, Preacquisition Contingency, Amount of Settlement		0	(6,587)	0	(6,587)	$ (6,587)
Issuance of common stock for stock-based compensation				9,622	3,709
Employee Tax Withholdings by settlement of net share transaction				(13,051)	(11,802)
Repurchase of common stock				(179,649)	(201,673)
Dividends paid				(47,639)	(44,103)
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders				27,076	13,061
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES				(238,169)	129,375
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Income Taxes Paid, Net				7,277	11,025
Adjustments to reconcile Net Income to net cash used in operations:
Unrealized Gain (Loss) on Investments				(10,800)	(21,382)
Net (gains) losses on investments				(7,037)	3,391
Decrease (increase) in assets:
Other assets				11,456	(11,290)
Increase (decrease) in liabilities:
Other liabilities				(21,237)	(37,733)
CASH FLOWS FROM INVESTING ACTIVITIES
Cash Acquired		33,547		33,547
Proceeds from sales and maturities of investment securities				5,506	2,436
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Lines of Credit				0	(40,000)
Debt issuance costs				0	(17,639)
Proceeds from Issuance of Long-term Debt				0	500,000
EFFECT OF EXCHANGE RATES ON CASH				(1,923)	248
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS				(79,475)	(758,258)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR				733,709	1,329,126	1,329,126
CASH AND CASH EQUIVALENTS AT END OF YEAR		654,234	570,868	654,234	570,868	733,709	$ 1,329,126
Interest Paid				56,670	45,485
Proceeds from Income Tax Refunds				(9,505)	(754)
Consolidated Investment Vehicles [Member]
Adjustments to reconcile Net Income to net cash used in operations:
Net (gains) losses on investments				(3,091)	(8,434)
Decrease (increase) in assets:
Other assets				(32,924)	56,672
Increase (decrease) in liabilities:
Other liabilities				(100)	2,101
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from (Payments to) Noncontrolling Interests				19,624	(39,469)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR				651
CASH AND CASH EQUIVALENTS AT END OF YEAR		$ 2,475		2,475		651
Change in Input Assumptions [Member] | Consolidated Legg Mason, Inc.
Adjustments to reconcile Net Income to net cash used in operations:
Contingent consideration fair value adjustments				(16,550)	(25,000)	$ (39,500)
Consolidated Investment Vehicles [Member]
CASH FLOWS FROM FINANCING ACTIVITIES
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders				0	0
Proceeds from (Payments to) Noncontrolling Interests	[1],[2]			$ 22,317	(42,073)
Management Equity Plan [Member] | Clarion Partners [Member]
Adjustments to reconcile Net Income to net cash used in operations:
Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense			$ 15,200		$ 15,200
5.5% Senior Notes [Member] | Consolidated Legg Mason, Inc.
CASH FLOWS FROM OPERATING ACTIVITIES
Payment for Debt Extinguishment or Debt Prepayment Cost							$ 107,074

[1] (1) Principally related to VIE and seeded investment products.

[2] (3) Includes the impact related to the adoption of updated consolidation accounting guidance.