Stock-Based Compensation (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Mar. 31, 2012		Mar. 31, 2011	Mar. 31, 2010	Jul. 26, 2011
Stock based compensation disclosure
Increase In Available Shares					6,500
Total shares available under equity incentive plan					41,500
Shares available for issuance under equity incentive plan (in shares)	13,134
Share-based Compensation Arrangement by Share-based Payment Award Granted Price as Percentage of Fair Market Value Minimum	100.00%
Share-based Compensation Option Exercisable Period from Date of Grant, Low End of Range	4
Share-based Compensation Option Exercisable Period from Date of Grant, High End of Range	5
Share-based Compensation Option Expiration Period from Date of Grant, Low End of Range	8
Share-based Compensation Option Expiration Period from Date of Grant, High End of Range	10
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 5,539,000		$ 7,718,000	$ 6,221,000
Summary of unvested stock options under equity incentive plans
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	398,000		6,977,000	229,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	1,715,000
Options exercisable (in shares)	3,334		2,860	2,810
Weighted-average exercise price per share of exercisable options (in dollars per share)	$ 73.60		$ 77.20	$ 73.57
Weighted-average remaining contractual life of exercisable options (in years)	2.9
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	934,000
Share-based Compensation Arrangement by Share-based Payment Award Maximum Number of Shares that can be Purchased for Award	4,500
Share-based Compensation Arrangement by Share-based Payment Award, Shares Purchased for Award Contribution Provided Percent	10.00%
Share-based Compensation Arrangement by Share-based Payment Award, Shares Purchased for Award	107		102	147
Employee Stock Ownership Plan (ESOP), Compensation Expense	267,000		286,000	313,000
Unvested options
Summary of unvested stock options under equity incentive plans
Unvested options, beginning of period	2,559
Unvested options, granted	810
Unvested Options, options vested in period	(961)	[1]
Unvested options, forfeitures in period	(118)
Unvested Options, end of period	2,290
Unvested options outstanding, weighted average exercise price, beginning of period	15.89
Unvested options, granted, weighted-average price	$ 13.13
Vested in period (in dollars per share)	$ 17.84	[1]
Canceled/ forfeited (in dollars per share)	$ 14.78
Unvested options outstanding, weighted average exercise price, end of period	14.00
Unamortized compensation cost related to unvested options	22,843,000
Weighted-average period over which compensation cost related to unvested options is expected to be recognized (in years)	1.7
Stock options
Summary of unvested stock options under equity incentive plans
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options	2,851,000		12,094,000	1,829,000
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options	$ 47,000		$ 2,645,000	$ 73,000
Grants (in dollars per share)	$ 13.13		$ 14.32	$ 12.09
Legg Mason, Inc.
Stock Option Transactions
Options outstanding, beginning of period (in shares)	5,419		6,054	5,554
Granted (in shares)	810		729	1,457
Exercised (in shares)	(117)		(634)	(72)
Forfeitures/cancelations (in shares)	(488)		(730)	(885)
Options outstanding, end of period (in shares)	5,624		5,419	6,054
Options outstanding, beginning of period (in dollars per share)	$ 59.82		$ 57.75	$ 64.09
Granted (in dollars per share)	$ 33.99		$ 33.12	$ 26.82
Exercised (in dollars per share)	$ 25.32		$ 21.85	$ 25.40
Canceled/forfeited (in dollars per share)	$ 48.80		$ 48.94	$ 49.24
Options outstanding, end of period (in dollars per share)	$ 57.78		$ 59.82	$ 57.75

[1] Stock options granted prior to fiscal 2011 vest in July each year; beginning in fiscal 2011, stock options granted vest in May each year.