Significant Accounting Policies (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Variable Interest Entity [Line Items]
Long-term Debt, Fair Value	$ 1,214,245	$ 1,322,960
Number of Operating Segments	1
Minimum Probability of Tax Benefit Recognition upon Settlement for Benefit to be Recorded	50.00%
Deferred Sales Commissions, Amortization Period	6
Deferred Sales Inducements, Net	$ 9,510	$ 11,339
Legg Mason, Inc. | Estimate of Fair Value, Fair Value Disclosure
Variable Interest Entity [Line Items]
Cash and Cash Equivalents, Maturity Period	90