Risk Management Activities and Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Fair value of risk management assets and liabilities
Short-term risk management assets	$ 39,316	$ 21,159
Long-term risk management assets	5,361	6,593
Short-term risk management liabilities	(16,290)	(20,005)
Long-term risk management liabilities	(5,098)	(4,574)
Total	23,289	3,173
Not designated as hedge
Fair value of risk management assets and liabilities
Short-term risk management assets	39,316	21,159
Long-term risk management assets	5,361	6,593
Short-term risk management liabilities	(16,290)	(20,005)
Long-term risk management liabilities	(5,098)	(4,574)
Total	23,289	3,173
Energy Contracts
Fair value of risk management assets and liabilities
Total	21,795	2,707
Energy Contracts | Not designated as hedge
Fair value of risk management assets and liabilities
Short-term risk management assets	38,153	20,383
Long-term risk management assets	5,017	6,593
Short-term risk management liabilities	(16,290)	(19,792)
Long-term risk management liabilities	(5,085)	(4,477)
Total	21,795	2,707
Currency contracts
Fair value of risk management assets and liabilities
Total	1,494	466
Currency contracts | Not designated as hedge
Fair value of risk management assets and liabilities
Short-term risk management assets	1,163	776
Long-term risk management assets	344
Short-term risk management liabilities		(213)
Long-term risk management liabilities	(13)	(97)
Total	$ 1,494	$ 466