Income Taxes Components of Income Tax Expense (Benefit) (Details) - Consolidated Legg Mason, Inc. - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Schedule of Income Tax Expense [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ 308,751	$ 245,046	$ 249,380
Income (Loss) from Continuing Operations before Income Taxes, Foreign	62,127	(270,264)	118,613
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	370,878	(25,218)	367,993
Federal Income Tax Expense (Benefit), Continuing Operations	68,336	87,166	95,499
Foreign Income Tax Expense (Benefit), Continuing Operations	2,535	(71,828)	20,365
State and Local Income Tax Expense (Benefit), Continuing Operations	13,304	(7,646)	9,420
Income Tax Expense (Benefit)	84,175	7,692	125,284
Current Income Tax Expense (Benefit)	26,371	15,419	24,897
Deferred Income Tax Expense (Benefit)	$ 57,804	$ (7,727)	$ 100,387