Business Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Revenues and long-lived assets by geographic region
Number of Reportable Segments									1
Legg Mason, Inc
Revenues and long-lived assets by geographic region
Operating Revenues	$ 681,396	$ 720,092	$ 669,852	$ 670,417	$ 667,763	$ 673,900	$ 640,295	$ 630,692	$ 2,741,757	$ 2,612,650	$ 2,662,574
Intangible Assets and Goodwill	4,412,296				4,446,727				4,412,296	4,446,727	5,131,911
Legg Mason, Inc | United States
Revenues and long-lived assets by geographic region
Operating Revenues									1,874,328	1,800,539	1,806,990
Intangible Assets and Goodwill	3,127,654				3,139,050				3,127,654	3,139,050	3,548,628
Legg Mason, Inc | United Kingdom
Revenues and long-lived assets by geographic region
Operating Revenues									436,542	387,966	448,863
Intangible Assets and Goodwill	879,946				895,767				879,946	895,767	1,108,297
Legg Mason, Inc | Other International
Revenues and long-lived assets by geographic region
Operating Revenues									430,887	424,145	406,721
Intangible Assets and Goodwill	$ 404,696				$ 411,910				$ 404,696	$ 411,910	$ 474,986