Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Other comprehensive income (loss):
Foreign currency translation adjustment	$ (90,103)	$ (59,672)
Consolidated Legg Mason, Inc.
NET INCOME	286,703	(32,910)	$ 242,709
Other comprehensive income (loss):
Foreign currency translation adjustment	(32,924)	(8,525)	(88,982)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax	2,493	0	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	(2,718)	0	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	1,708	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	2,718	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	1,708	0	0
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	0	0	0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0	(5)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	0	0	3
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	0	0	(3)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax	0	0	(114)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(9,860)	2,774	(9,595)
Total other comprehensive income (loss)	(40,291)	(5,751)	(98,691)
COMPREHENSIVE INCOME	246,412	(38,661)	144,018
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	59,600	(11,738)	5,629
COMPREHENSIVE INCOME ATTRIBUTABLE TO LEGG MASON, INC.	186,812	(26,923)	138,389
AOCI Attributable to Parent [Member] | Consolidated Legg Mason, Inc.
Other comprehensive income (loss):
Foreign currency translation adjustment	(32,924)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax	(2,493)	0	0
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax	0	0	(114)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(9,860)	2,774	(9,595)
Treasury Lock [Member] | Consolidated Legg Mason, Inc.
Other comprehensive income (loss):
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	0	0	405
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0		(233)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	$ 0	(405)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	$ 0		$ (233)