Long-Term Debt (Details 3) (USD $)		12 Months Ended			1 Months Ended	12 Months Ended					12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended			12 Months Ended
	Mar. 31, 2012	Mar. 31, 2012 Legg Mason, Inc.	Mar. 31, 2011 Legg Mason, Inc.	Mar. 31, 2010 Legg Mason, Inc.	Jan. 31, 2008 2.5% Convertible Senior Notes [Member]	Mar. 31, 2012 2.5% Convertible Senior Notes [Member]	Mar. 31, 2011 2.5% Convertible Senior Notes [Member]	Mar. 31, 2010 2.5% Convertible Senior Notes [Member]	May 23, 2012 2.5% Convertible Senior Notes [Member]	Jan. 14, 2008 2.5% Convertible Senior Notes [Member]	Mar. 31, 2012 2.5% Convertible Senior Notes [Member] Legg Mason, Inc.	May 31, 2008 5.6% senior notes from Equity Units	Sep. 30, 2009 5.6% senior notes from Equity Units	Mar. 31, 2012 5.6% senior notes from Equity Units	Mar. 31, 2010 5.6% senior notes from Equity Units	Mar. 31, 2011 5.6% senior notes from Equity Units	Mar. 31, 2012 5.6% senior notes from Equity Units Legg Mason, Inc.	Jun. 30, 2011 5.6% senior notes from Equity Units Legg Mason, Inc.	Mar. 31, 2011 5.6% senior notes from Equity Units Legg Mason, Inc.	Mar. 31, 2012 Aircraft	Mar. 31, 2011 Aircraft	Mar. 31, 2006 Aircraft	May 23, 2012 Senior Notes due May 2019 [Member]
Debt Instruments
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 for 2.5% Convertible Senior Notes		$ 39,077,000	$ 36,688,000	$ 34,445,000		$ 39,077,000	$ 36,688,000	$ 34,445,000
Debt Instrument, Convertible, Conversion Ratio						11.3636
Debt Instrument, Convertible, Conversion Price						$ 88
Maximum shares issuable upon conversion (in shares)						14,205,000
Amount by which the if-converted value of the Notes exceeds their accreted value						77,353,000
Percentage of interest rate used in the calculation of the if-converted value of convertible debt (as a percent)						4.00%
Maximum gross debt to EBITDA ratio						2.5
Actual gross debt to EBITDA ratio						2.7
Class of Warrant or Right, Number of Securities Called by Warrants or Rights						14,205,000
Class of Warrant or Right, Exercise Price of Warrants or Rights						$ 107.46
Payments for Hedge, Financing Activities					83,000,000
Number of Equity Units Issued												23,000,000
Proceeds from issuance of equity units (in dollars)												1,150,000,000
Debt Issue Costs		0	0	3,056,000								50,000,000
Percent of interest in $1,000 principal amount of senior notes per equity unit issued (as a percent)														5.00%
Debt Instrument, Convertible Conversion Ratio, Face Amount of Note														1,000
Purchase Contract Stock Price Per Share														$ 50
Debt Issuance Costs Incurred During Noncash or Partial Noncash Transaction												36,200,000
Adjustments to Additional Paid in Capital Allocated Debt Issuance Cost												27,600,000
Fixed Annual Rate of Contract Adjustment Payments														1.40%
Fair Value upon Issuance of Obligation to Pay Unitholders Quarterly Contract Adjustment Payment												45,800,000
Contract Adjustment Payments Obligation Liability																168,000
Equity Unit Settlement Rate Common Stock Price, Low End of Range														$ 56.30
Equity Unit Settlement Rate, Common Stock Price, High End of Range														$ 67.56
Settlement Rate under Purchase Contract if Stock Price is 56.30 or below														0.8881
Settlement Rate under Purchase Contract if Stock Price is 67.56 or above														0.7401
Percentage of equity units exchanged (as a percent)													91.00%
Number of Equity Units Retired													20,939,000
Extinguishment of Debt, Amount													1,050,000,000
Equity Units exchange		1,830,000	0	18,596,000											18,596,000		1,830,000
Payments on Equity Units Exchange, Net													130,870,000
Payments on Equity Units Exchange		0	0	135,015,000
Loss on Equity Units exchange													22,040,000
Write off of Deferred Debt Issuance Cost													6,355,000
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments													115,186,000
Outstanding debt from senior notes																			103,039,000
Conversion of note, cash settlement option														1,000
Term for repayment of long-term debt														3
Principal amount of debt issued (in dollars)										1,250,000,000												12,803,000	650,000,000
Other Long-term Debt, Noncurrent																				8,568,000	9,363,000
Senior Notes Repurchased									$ 1,250,000,000														$ 1,250,000,000
Debt instrument interest rate stated percentage (as a percent)	2.50%								2.50%	2.50%								5.60%				5.90%	5.50%