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INVESTMENTS AND FAIR VALUES OF ASSETS AND LIABILITIES (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2011	Mar. 31, 2011 Legg Mason, Inc. Quoted prices in active markets (Level 1)	Mar. 31, 2010 Legg Mason, Inc. Quoted prices in active markets (Level 1)	Mar. 31, 2011 Legg Mason, Inc. Significant other observable inputs (Level 2)	Mar. 31, 2010 Legg Mason, Inc. Significant other observable inputs (Level 2)	Mar. 31, 2011 Legg Mason, Inc. Significant unobservable inputs (Level 3)	Mar. 31, 2010 Legg Mason, Inc. Significant unobservable inputs (Level 3)	Mar. 31, 2011 Legg Mason, Inc. Total	Mar. 31, 2010 Legg Mason, Inc. Total	Mar. 31, 2011 Legg Mason, Inc.	Mar. 31, 2010 Legg Mason, Inc.
Cash equivalents
Cash and cash equivalents maximum maturity period (in days)	90
Money market funds		$ 912,951	$ 930,015					$ 912,951	$ 930,015
Time deposits				92,877	249,352			92,877	249,352
Total cash equivalents		912,951	930,015	92,877	249,352			1,005,828	1,179,367
Investment securities:
Trading investments relating to long-term incentive compensation plans		120,107	118,096					120,107	118,096
Trading proprietary fund products and other investments		90,123	52,375	102,562	67,663	11,378	22,459	204,063	142,497
Equity method investments relating to long-term incentive compensation plans, proprietary fund products and other investments		15,645	13,159	48,528	49,031	12,167	12,090	76,340	74,280
Total current investments		225,875	183,630	151,090	116,694	23,545	34,549	400,510	334,873
Available-for-sale investment securities		2,666	2,533	8,622	4,412	12	12	11,300	6,957	11,300	6,957
Investments in partnerships and, LLCs, and other						22,167	23,049	22,167	23,049
Equity method investments in partnerships and LLCs		1,420	1,192			153,931	98,968	155,351	100,160
Derivative assets, Currency and market hedges		1,169	697					1,169	697
Other investments						270	1,452	270	1,452	270	1,452
Financial assets measured at fair value		1,144,081	1,118,067	252,589	370,458	199,925	158,030	1,596,595	1,646,555
Derivative liabilities:
Currency and market hedges		(3,120)	(485)					(3,120)	(485)
Proprietary fund products disclosures
Proprietary fund products and other investments, equity securities percentage (as a percent)										60.00%	63.00%
Proprietary fund products and other investments, debt securities percentage (as a percent)										40.00%	37.00%
Equity method investments relating to long-term incentive compensation plans										48,528	49,031
Equity method investments relating to proprietary fund products and other investments										$ 27,812	$ 25,249