Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Nov. 30, 2013	Nov. 30, 2012
OPERATING ACTIVITIES
Net loss	$ (17,210)	$ (13,778)
Adjustments to reconcile net loss to cash provided by operating activities
Depreciation, depletion and amortization	39,433	28,543
Gain on asset disposals	(5,766)	(2,879)
Deferred income tax expense (benefit)	(9)	957
Stock-based compensation expense	3,962	5,505
Other – net	(7,703)	(8,469)
Changes in operating assets and liabilities
Receivables – net	4,401	(8,488)
Inventories	(7,720)	13,967
Prepaid expenses	1,949	1,687
Accounts payable and accrued liabilities	21,868	7,933
Net cash provided by operating activities	33,205	24,978
INVESTING ACTIVITIES
Capital expenditures – expansions	(7,125)	(36,118)
Capital expenditures – other	(17,295)	(10,845)
Proceeds from asset disposals	7,682	3,958
Investments in life insurance contracts	4,059	2,366
Other – net	0	(88)
Net cash used by investing activities	(12,679)	(40,727)
FINANCING ACTIVITIES
Debt payments	(910)	(1,585)
Stock option exercises	1,450	1,089
Net cash provided (used) by financing activities	540	(496)
Increase (decrease) in cash and cash equivalents	21,066	(16,245)
Cash and cash equivalents at beginning of period	61,296	88,027
Cash and cash equivalents at end of period	$ 82,362	$ 71,782