Short-Term Borrowings and Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified							12 Months Ended
	Mar. 31, 2013 Legg Mason Group	May 21, 2012 5.5% senior notes Legg Mason Group	May 21, 2012 5.5% senior notes Legg Mason, Inc.	Jun. 27, 2012 Previous revolving credit facility Legg Mason Group	May 23, 2012 Previous revolving credit facility Legg Mason Group	Mar. 31, 2012 Previous revolving credit facility Legg Mason Group	Mar. 31, 2013 New revolving credit facility Legg Mason Group	Jun. 27, 2012 New revolving credit facility Legg Mason Group	Mar. 31, 2013 Five-year amortizing term loan Legg Mason Group	Jun. 27, 2012 Five-year amortizing term loan Legg Mason Group
Debt Instrument
Issuance of long-term debt		$ 650,000								$ 500,000
Debt Instrument, Interest Rate, Stated Percentage		5.50%
Additional borrowings under revolving credit facility					250,000
Line of Credit Facility, Repayment of Borrowings				500,000
Line of Credit Facility, Remaining Borrowing Capacity								500,000
Outstanding borrowings under credit facility						250,000
Effective interest rate for revolving credit agreement (as a percent)						2.90%
Line of Credit Facility, Maximum Borrowing Capacity								500,000
Letter of Credit Facility, Potential Increase in Maximum Borrowing Capacity								250,000
Debt Instrument, Basis Spread on Variable Rate								1.50%		1.50%
Line of Credit Facility, Commitment Fee Percentage							0.20%
Maximum net debt to EBITDA ratio								2.5		2.5
Minimum EBITDA to interest ratio								4.0		4.0
Net debt to EBITDA ratio							1.2		1.2
EBITDA to interest expense ratio							12.6		12.6
Debt Instrument, Periodic Payment, Principal										50,000
Debt Instrument, Interest Rate at Period End									1.70%
Number of annual debt installment payments										4
Term of loan in years		7
Discount on notes	$ 5,923	$ 6,754
Contingent prepayment fee			0.50%