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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		$ 11,983	$ 7,989	$ 16,059	$ 8,528
Derivative Instruments Not Designated as Hedging Instruments, Loss		(17,841)	(9,227)	(19,940)	(12,298)
Derivative, Gain on Derivative		14,755	8,627	17,856	9,166
Derivative, Loss on Derivative		(17,841)	(10,523)	(19,940)	(12,847)
Forward Contracts [Member] | Other Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		1,404	1,568	4,209	2,219
Derivative Instruments Not Designated as Hedging Instruments, Loss		(6,722)	(6,938)	(5,822)	(5,165)
Forward Contracts [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		546	881	326	749
Derivative Instruments Not Designated as Hedging Instruments, Loss		(55)	(23)	(402)	(211)
Future [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain		10,033	5,540	11,524	5,560
Derivative Instruments Not Designated as Hedging Instruments, Loss		0	(2,266)	(2,652)	(6,922)
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative		2,772	0	1,797	0
Derivative, Loss on Derivative		0	(1,296)	0	(549)
Treasury Lock [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain on Derivative		0	638	0	638
Derivative, Loss on Derivative		0	0	0	0
RARE Infrastructure, Ltd [Member] | Foreign Exchange Forward [Member] | Other Nonoperating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain	[1]	0	0	0	0
Derivative Instruments Not Designated as Hedging Instruments, Loss	[1]	$ (11,064)	$ 0	$ (11,064)	$ 0

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