Fair Values of Assets and Liabilities (Details 3) (Legg Mason, Inc., USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	$ 199,925	$ 158,030
Purchases, sales, issuances, settlements, net		12,672
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	32,815
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(18,293)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(14,905)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	350
Realized and unrealized gains/(losses), net, assets	7,561	28,873
Assets measured at fair value using significant unobservable inputs, value at end of period	207,103	199,925
Unrealized gains (losses) on Level 3 assets and liabilities still held at the reporting date	5,495	11,472
Trading proprietary fund products and other investments
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	11,378	22,459
Purchases, sales, issuances, settlements, net		(13,429)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(11,906)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	350
Realized and unrealized gains/(losses), net, assets	528	1,998
Assets measured at fair value using significant unobservable inputs, value at end of period	0	11,378
Equity method investments in proprietary fund products
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	12,167	12,090
Purchases, sales, issuances, settlements, net		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net, assets	(389)	77
Assets measured at fair value using significant unobservable inputs, value at end of period	11,778	12,167
Investments in partnerships, LLCs and other
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	22,167	23,049
Purchases, sales, issuances, settlements, net		831
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	6,932
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(578)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net, assets	242	(1,713)
Assets measured at fair value using significant unobservable inputs, value at end of period	28,763	22,167
Equity method investments in partnerships and LLCs
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	153,931	98,968
Purchases, sales, issuances, settlements, net		29,335
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	25,883
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(6,387)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(14,168)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net, assets	7,179	25,628
Assets measured at fair value using significant unobservable inputs, value at end of period	166,438	153,931
Other Investments
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	282	1,464
Purchases, sales, issuances, settlements, net		(4,065)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(159)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net, assets	1	2,883
Assets measured at fair value using significant unobservable inputs, value at end of period	$ 124	$ 282