Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Other comprehensive income (loss):
Foreign currency translation adjustment		$ (41,547)	$ (90,103)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax			(2,493)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			2,718
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax				$ (405)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax			1,708
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax		492
Consolidated Legg Mason, Inc.
NET INCOME		336,350	286,703	(32,910)
Other comprehensive income (loss):
Foreign currency translation adjustment		48,556	(32,924)	(8,525)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax		0	2,493	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		0	(2,718)	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	$ 0	0	1,708
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		0	2,718	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	$ 0	0	1,708
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax		3,046	(9,860)	2,774
Total other comprehensive income (loss)		51,602	(40,291)	(5,751)
COMPREHENSIVE INCOME		387,952	246,412	(38,661)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest		50,894	59,600	(11,738)
COMPREHENSIVE INCOME ATTRIBUTABLE TO LEGG MASON, INC.		337,058	186,812	(26,923)
AOCI Attributable to Parent [Member] | Consolidated Legg Mason, Inc.
Other comprehensive income (loss):
Foreign currency translation adjustment		48,556
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax		0	(2,493)	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax		$ 3,046	$ (9,860)	$ 2,774