Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income
CASH FLOWS FROM FINANCING ACTIVITIES
Payment for Contingent Consideration Liability, Financing Activities					$ (4,319)	$ (2,503)
Proceeds from (Payments to) Noncontrolling Interests					(36,328)	44,708
Consolidated Legg Mason, Inc.
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	(206,390)	168,546			(46,952)	319,707
Adjustments to reconcile Net Income to net cash used in operations:
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	365,200	195,000			365,200	229,000
Depreciation and amortization					54,031	54,551
Accretion and amortization of securities discounts and premiums, net					1,591	2,255
Stock-based compensation					49,975	55,002
Deferred income taxes					(19,634)	46,650
Contingent consideration fair value adjustments	0	739			571	(15,811)
Other					286	(196)
Decrease (increase) in assets:
Investment advisory and related fees receivable					38,170	(47,603)
Net (purchases) sales of trading and other current investments					20,375	38,622
Other receivables					(16,555)	5,471
Increase (decrease) in liabilities:
Accrued compensation					51,796	(97,419)
Deferred compensation					(10,464)	10,633
Accounts payable and accrued expenses					31,130	15,278
CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES					443,989	291,818
CASH FLOWS FROM INVESTING ACTIVITIES
Payments for fixed assets					38,062	(24,540)
Payments to Acquire Other Investments					0	(2,950)
Proceeds from Sales of Business, Affiliate and Productive Assets					923	2,561
CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES					(26,928)	(12,364)
CASH FLOWS FROM FINANCING ACTIVITIES
Business Acquisition, Preacquisition Contingency, Amount of Settlement					0	(739)
Issuance of common stock for stock-based compensation					6,029	11,050
Employee Tax Withholdings Related to net share transaction					(15,547)	(13,074)
Repurchase of common stock	(253,649)	(74,000)				(479,139)
Dividends paid					(84,630)	(76,000)
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders					32,358	(43,477)
CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES					(309,181)	(332,935)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents					(16,470)	5,641
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect					91,410	(47,840)
Cash, Cash Equivalents, and Restricted Cash, Beginning of Period				$ 754,339		754,339	$ 754,339
Cash, Cash Equivalents, and Restricted Cash, End of Period	865,175	706,499			865,175	706,499
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Restricted Cash and Cash Equivalents, Noncurrent	4,143	4,247			4,143	4,247
Restricted Cash and Cash Equivalents, Current	$ 22,336	$ 17,605			22,336	17,605	30,428
Income Taxes Paid, Net					30,542	16,770
Adjustments to reconcile Net Income to net cash used in operations:
Net unrealized losses (gains) on investments					40,446	(21,841)
Net losses (gains) on investments					(36,495)	(2,874)
Decrease (increase) in assets:
Other assets					(6,282)	2,199
Increase (decrease) in liabilities:
Other liabilities					(83,376)	(24,853)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from Sale, Maturity and Collection of Investments					10,211	12,565
CASH FLOWS FROM FINANCING ACTIVITIES
Interest Paid, Including Capitalized Interest, Operating and Investing Activities					69,748	67,482
Proceeds from Income Tax Refunds			$ (9,505)
Consolidated Investment Vehicles [Member]
Adjustments to reconcile Net Income to net cash used in operations:
Net losses (gains) on investments					(1,954)	(11,316)
Decrease (increase) in assets:
Other assets					(751)	(50,974)
Increase (decrease) in liabilities:
Other liabilities					10,717	(249)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from (Payments to) Noncontrolling Interests					(36,328)	44,708
RARE Infrastructure, Ltd [Member]
CASH FLOWS FROM FINANCING ACTIVITIES
Payments to Noncontrolling Interests					(16,528)
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					571	(15,811)	$ (31,329)
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				$ 47,401	(36,328)
Short-term Debt [Member] | Consolidated Legg Mason, Inc.
CASH FLOWS FROM FINANCING ACTIVITIES
Repayments of Debt					$ (125,500)
Proceeds from Issuance of Debt						$ 225,500