Supplemental Cash Flow Disclosures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Supplemental Cash Flow Disclosures
Interest paid	$ 63,769	$ 67,408	$ 75,375
Taxes paid (received)	73	(722)	988
Interest capitalized		2,928	4,077
Non-cash investing activities:
Non-cash (decrease) increase in working capital related to property, plant and equipment	(2,426)	(8,605)	11,296
Non-cash transfer of natural gas inventory to property, plant and equipment	15,264
Non-cash financing activities:
Non cash earnings distribution and reinvestments	$ 18,270