Derivatives and Hedging Gains and (Losses) of Derivative Instruments (Details) - Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] - USD ($) $ in Thousands	1 Months Ended	6 Months Ended	12 Months Ended
	Apr. 16, 2016	Sep. 30, 2016	Mar. 31, 2017	Mar. 31, 2016		Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			$ 19,308	$ 19,704		$ 18,442
Derivative Instruments Not Designated as Hedging Instruments, Loss			(34,143)	(34,857)		(32,190)
Derivative, Gain on Derivative			19,308	25,414
Derivative, Loss on Derivative			(38,569)	(34,857)
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative	$ 6,500
Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative			0	0
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative			0	5,710		5,462
Derivative, Loss on Derivative				0		0
Treasury Lock [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative						638
Derivative, Loss on Derivative						0
Forward Contracts [Member] | Other Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			14,524	7,887		5,150
Derivative Instruments Not Designated as Hedging Instruments, Loss			(13,098)	(19,547)		(16,518)
Forward Contracts [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			0	0		0
Derivative Instruments Not Designated as Hedging Instruments, Loss			0	(4,493)	[1]	0	[1]
Future [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain			2,103	11,270		10,801
Derivative Instruments Not Designated as Hedging Instruments, Loss			(18,602)	(9,206)		(15,413)
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)	0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Loss on Derivative			(764)
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			2,681	547		2,491
Derivative Instruments Not Designated as Hedging Instruments, Loss			$ (2,443)	$ (1,611)		$ (259)

[1] Relates to a currency forward executed in August 2015 and closed in October 2015 in connection with the October 2015 acquisition of RARE Infrastructure.