Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
OPERATING ACTIVITIES
Net income (loss)	$ 24,550	$ 7,476	$ (64,913)
Adjustments to reconcile net income (loss) to cash provided by operating activities
Depreciation, depletion and amortization	59,865	60,952	64,297
(Gains), net on asset disposals	(64,425)	(67,610)	(13,638)
Deferred income tax (benefit) expense	3,423	(88)	(42,875)
Stock-based compensation expense	9,513	2,387	5,581
Loss on debt retirements	0	0	29,619
Other – net	(6,965)	1,223	3,158
Changes in operating assets and liabilities
Receivables – net	(27,138)	(13,303)	13,379
Inventories	21,433	10,829	2,164
Prepaid expenses	(238)	1,385	1,301
Accounts payable and accrued liabilities	13,282	6,923	11,172
Net cash provided by operating activities	33,300	10,174	9,245
INVESTING ACTIVITIES
Capital expenditures – expansions	(67,426)	(72,906)	(25,430)
Capital expenditures – other	(25,395)	(33,430)	(20,253)
Proceeds from asset disposals	18,481	66,845	3,596
Investments in life insurance contracts	2,467	3,354	4,073
Other – net	(102)	(245)	1,266
Net cash provided (used) by investing activities	(71,975)	(36,382)	(36,748)
FINANCING ACTIVITIES
Long-term borrowings	0	0	650,000
Debt payments	(2,684)	(300)	(561,627)
Debt issuance costs	0	(1,829)	(12,492)
Stock option exercises	14,628	2,023	1,462
Excess tax benefits from stock-based compensation	0	0	0
Common dividends paid	0	(2,091)	(8,354)
Net cash provided (used) by financing activities	11,944	(2,197)	68,989
Increase (decrease) in cash and cash equivalents	(26,731)	(28,405)	41,486
Cash and cash equivalents at beginning of period	88,027	116,432	74,946
Cash and cash equivalents at end of period	$ 61,296	$ 88,027	$ 116,432