Short-Term Borrowings and Long-Term Debt (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified			3 Months Ended		9 Months Ended												3 Months Ended
	Dec. 31, 2011 2.5% Convertible Senior Notes	Dec. 31, 2012 Legg Mason Group	Dec. 31, 2012 Legg Mason Group 2.5% Convertible Senior Notes	Dec. 31, 2011 Legg Mason Group 2.5% Convertible Senior Notes	Dec. 31, 2012 Legg Mason Group 2.5% Convertible Senior Notes	Dec. 31, 2011 Legg Mason Group 2.5% Convertible Senior Notes	May 23, 2012 Legg Mason Group 2.5% Convertible Senior Notes	Dec. 31, 2012 Legg Mason Group Five-year term loan	Dec. 31, 2012 Legg Mason Group 5.5% Senior Notes	May 23, 2012 2.5% Convertible Senior Notes Legg Mason Group	Dec. 31, 2012 5.5% Senior Notes 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	Dec. 31, 2012 New revolving credit facility Legg Mason Group	Jun. 27, 2012 New revolving credit facility Legg Mason Group	Dec. 31, 2012 Five-year term loan Legg Mason Group	Jun. 27, 2012 Five-year term loan Legg Mason Group
Debt Instrument
Debt Instrument, Interest Rate, Stated Percentage	2.50%									2.50%	6.00%	5.50%
Issuance of long-term debt												$ 650,000								$ 500,000
Line of Credit Facility, Repayment of Borrowings														500,000
Additional borrowings under revolving credit facility															250,000
Senior Notes Repurchased										1,250,000
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%
Additional Interest over Stated Rate due to Noncompliance with Registration Rights - Maximum											0.50%	0.50%
Additional Interest over Stated Rate due to Noncompliance with Registration Rights - Current											0.50%	0.50%
Contingent prepayment fee													0.50%
Debt Instrument, Periodic Payment, Principal																				50,000
Debt Instrument, Interest Rate at Period End																			1.70%
Number of annual debt installment payments																				4
Unamortized Discount		6,164	0		0			0	6,164			6,754
Term of loan in years												7
Debt Instrument, Fee			6,250		6,250
Cash payment to repurchase senior notes							1,256,250
Fair value at repuchase							1,193,971
Debt instrument, current market interest rate							4.10%
Accelerated deferred issue costs			7,851
Adjustments to Additional Paid in Capital			62,279
Extinguishment of Debt, related tax benefit			31,446
Underlying shares issuable upon exercise of warrants							14,205
Exercise price of warrants							$ 88
Fair value of convertible debt allocated to liability component at issuance							977,933
Excess of allocation to liability component of convertible debt at extinguishment over inital allocation			216,038
Debt instrument, effective borrowing rate used to accrete carrying value (as a percent)			6.50%
Debt instrument term for computation of effective rate (in years)			7
Imputed interest				$ 9,793	$ 5,839	$ 29,023
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