Risk Management Activities and Financial Instruments (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Unrealized	$ 20,118	$ (51,148)
Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Total	15,240	(22,044)
Energy Contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	(6,342)	28,653
Unrealized	19,090	(52,578)
Currency contracts | Not designated as hedge
Net realized and unrealized optimization gains and losses from the settlement of risk management contracts
Realized	1,464	451
Unrealized	$ 1,028	$ 1,430