Earnings Per Share Schedule of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Consolidated Legg Mason, Inc.
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted Average Number of Shares Outstanding, Basic	93,087	101,817	93,973	103,075
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	409	240	417	226
Weighted Average Number of Shares Outstanding, Diluted	93,496	102,057	94,390	103,301
Net Income Attributable to Legg Mason, Inc.	$ 75,664	$ 66,441	$ 126,584	$ 99,893
Basic (in dollars per share)	$ 0.78	$ 0.63	$ 1.30	$ 0.94
Earnings Per Share, Diluted	$ 0.78	$ 0.63	$ 1.29	$ 0.94
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income Attributable to Legg Mason, Inc.	$ 75,664	$ 66,441	$ 126,584	$ 99,893
Participating Securities, Distributed and Undistributed Earnings (Loss), Diluted	2,687	2,183	4,387	3,173
Net Income (Loss) Available to Common Stockholders, Diluted	$ 72,977	$ 64,258	$ 122,197	$ 96,720
Restricted Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights	3,417	3,447	3,305	3,291