Risk Management Activities and Financial Instruments (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Unrealized			$ 6,486	$ (79,856)
Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Total	532	(26,248)	15,772	(48,432)
Energy Contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	14,032	2,882	7,690	31,435
Unrealized	(12,234)	(26,513)	6,856	(79,131)
Currency contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	132	(439)	1,596	12
Unrealized	$ (1,398)	$ (2,178)	$ (370)	$ (748)