Earnings Per Share Schedule of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Consolidated Legg Mason, Inc.
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted Average Number of Shares Outstanding, Basic	100,580	107,406	112,019
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	219	0	1,227
Weighted Average Number of Shares Outstanding, Diluted	100,799	107,406	113,246
Net Income Attributable to Legg Mason, Inc.	$ 227,256	$ (25,032)	$ 237,080
Basic (in dollars per share)	$ 2.19	$ (0.25)	$ 2.06
Earnings Per Share, Diluted	$ 2.18	$ (0.25)	$ 2.04
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.	$ 227,256	$ (25,032)	$ 237,080
Participating Securities, Distributed and Undistributed Earnings (Loss), Diluted	7,384	2,288	6,340
Net Income (Loss) Available to Common Stockholders, Diluted	$ 219,872	$ (27,320)	$ 230,740
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	3,335	2,831	3,065