Significant Accounting Policies (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Dec. 31, 2012
Accounting Policies [Abstract]
Business Acquisition, Initial Cash Payment	$ 80,000
Business Acquisition, Cash Payment Due on Second Anniversary	24,000
Business Acquisition, Cash Payment Due on Fourth Anniversary	$ 32,000
Profits interests equity plan participation percentage	15.00%

Represents the percentage of participation in future growth in value of the affiliate available for participation by key employees under the terms of a profits interest management equity plan.

No definition available.

Name:	lm_ProfitsInterestsEquityPlanParticipationPercentage
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration