Derivatives and Hedging Gains and (Losses) of Derivative Instruments (Details) - Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	$ 6,317	$ 2,190	$ 10,819	$ 15,185
Derivative, Loss on Derivative	(11,955)	(11,443)	(19,447)	(23,580)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	6,317	2,190	10,819	15,185
Derivative Instruments Not Designated as Hedging Instruments, Loss	(11,955)	(7,544)	(19,447)	(19,154)
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member] | Other Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	4,989	1,824	9,440	12,177
Derivative Instruments Not Designated as Hedging Instruments, Loss	(1,926)	1,477		9,283
Not Designated as Hedging Instrument [Member] | Total Return Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	885	0	778	0
Derivative Instruments Not Designated as Hedging Instruments, Loss	0	0	0	0
Not Designated as Hedging Instrument [Member] | 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	69	366		1,256
Derivative Instruments Not Designated as Hedging Instruments, Loss	(1,153)	748		1,160
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative		0		0
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	0	0	0	0
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative		(3,662)		(3,662)
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative	0	(237)	0	(764)
Investments [Member] | Not Designated as Hedging Instrument [Member] | Future [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	285	0	286	1,752
Derivative Instruments Not Designated as Hedging Instruments, Loss	(6,813)	(5,319)	(11,465)	(8,711)
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Future [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	89	0	89	0
Derivative Instruments Not Designated as Hedging Instruments, Loss	$ 2,063	$ 0	$ 1,797	$ 0