Earnings and Dividends Per Share Information about Share Repurchase Plan (Details) - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
	Dec. 22, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	5,568	6,636		11,697
Stock Repurchased and Retired During Period, Value	$ 225,490		$ 253,649	$ 381,672
Shares Paid for Tax Withholding for Share Based Compensation		395	348	370
Adjustments Related to Tax Withholding for Share-based Compensation		$ 15,575	$ 13,233	$ 12,139
Weighted Average Number Diluted Shares Outstanding Adjustment		355	6,050	6,335
Stock Repurchased During Period, Shares		0	12,204	11,697
Consolidated Legg Mason, Inc.
Class of Stock [Line Items]
Employee Tax Withholdings Related to net share transaction		$ 15,575	$ 13,233	$ 12,139
Repurchase of common stock			$ (479,139)	$ (381,672)