Fair Value Measurements (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Unrealized	$ (8,732)	$ (89,851)	$ 83,193
Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Total	(84,333)	(47,170)	136,359
Energy Contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	(78,778)	41,304	59,264
Unrealized	(10,662)	(90,641)	77,132
Currency contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	3,177	1,377	(6,064)
Unrealized	$ 1,930	$ 790	$ 6,027