Significant Accounting Policies Income Tax Policy (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2021	Mar. 31, 2018
Reconciliation of Tax Rate [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Other, Amount	$ 55,842
Effective Income Tax Rate Reconciliation, Percent					20.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					$ 8,361
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount	7,216	$ 7,026
Effective Income Tax Rate Reconciliation, Tax Contingency, Other, Amount	$ 8,479
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount			$ 17,527
Current Income Tax Expense (Benefit)				$ 4,300
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	3.70%				1.80%
Consolidated Legg Mason, Inc.
Reconciliation of Tax Rate [Line Items]
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 371,000			$ 0	$ 371,000	$ 0
Scenario, Forecast [Member]
Reconciliation of Tax Rate [Line Items]
Effective Income Tax Rate Reconciliation, Percent							18.00%	19.00%