Derivatives and Hedging Gains and (Losses) of Derivative Instruments (Details) - Not Designated as Hedging Instrument [Member] - Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	$ 7,872	$ 4,364	$ 10,938	$ 17,692
Derivative Instruments Not Designated as Hedging Instruments, Loss	(9,490)	(11,809)	(17,448)	(26,312)
Forward Contracts [Member] | Other Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	2,339	2,052	4,612	6,518
Derivative Instruments Not Designated as Hedging Instruments, Loss	(6,970)	(3,953)	(11,088)	(14,038)
Futures Related to Total Return Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	194	29	142	502
Derivative Instruments Not Designated as Hedging Instruments, Loss	(217)	(1,389)	(967)	(3,149)
Total Return Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	445	392	1,574	1,665
Derivative Instruments Not Designated as Hedging Instruments, Loss	(120)	(530)	0	(105)
Proprietary Funds [Member] | Forward Contracts [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	3,274	970	3,379	4,993
Derivative Instruments Not Designated as Hedging Instruments, Loss	(744)	(121)	(688)	(645)
Investments [Member] | Future [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	1,620	921	1,231	4,014
Derivative Instruments Not Designated as Hedging Instruments, Loss	$ (1,439)	$ (5,816)	$ (4,705)	$ (8,375)