Variable Interest Entities and Consolidation of Investment Vehicles Information about Investments in CIV's (Details) $ in Thousands		Dec. 31, 2018 USD ($)		Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entity
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	[1]	$ 71,564		$ 43,854	$ 41,548
Foreign Mutual Funds [Member]
Variable Interest Entity
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		49,251	[1]	12,485	10,114	[1]
Sponsored Investment Fund [Member]
Variable Interest Entity
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		12,863	[1]	16,670	16,537	[1]
Employee Owned Funds [Member]
Variable Interest Entity
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		6,963	[1]	7,328	7,107	[1]
Sponsored ETF [Member]
Variable Interest Entity
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		$ 2,487	[1]	$ 7,371	$ 7,790	[1]
Consolidated Investment Vehicles [Member] | Foreign Mutual Funds [Member]
Variable Interest Entity
Number of Funds		8		4	3
Consolidated Investment Vehicles [Member] | Sponsored Investment Fund [Member]
Variable Interest Entity
Number of Funds		2		2	2
Consolidated Investment Vehicles [Member] | Employee Owned Funds [Member]
Variable Interest Entity
Number of Funds		2		2	2
Consolidated Investment Vehicles [Member] | Sponsored ETF [Member]
Variable Interest Entity
Number of Funds	[2]	2		2	2
Future [Member]
Variable Interest Entity
Derivative, Notional Amount		$ 41,319

[1] (1) Represents Legg Mason's maximum risk of loss, excluding uncollected advisory fees.

[2] (2)Under the total return swap arrangements, Legg Mason receives the related investment gains and losses on investments in two of Legg Mason's ETFs with notional amounts totaling $41,319 as of December 31, 2018. See Note 12 for additional information regarding total return swaps.