Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	$ 10,174	$ 9,245	$ 48,153
Capital expenditures-expansions	(72,906)	(25,430)	(5,337)
Capital expenditures-other	(33,430)	(20,253)	(8,322)
Proceeds from asset disposals	66,845	3,596	21,592
Investments in life insurance contracts	3,354	4,073	6,967
Other-net	(245)	1,266	2,079
Net cash provided (used) by investing activities	(36,382)	(36,748)	16,979
Long-term borrowings		650,000
Debt retirements	(300)	(561,627)	(245)
Debt issuance costs	(1,829)	(12,492)	(2,552)
Stock option exercises	2,023	1,462	893
Excess tax benefits from stock-based compensation			250
Common dividends paid	(2,091)	(8,354)	(8,328)
Net cash provided (used) by financing activities	(2,197)	68,989	(9,982)
Increase (decrease) in cash and cash equivalents	(28,405)	41,486	55,150
Cash and cash equivalents at beginning of year	116,432	74,946	19,796
Cash and cash equivalents at end of year	88,027	116,432	74,946
Parent Company
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	(49,100)	(55,280)	(37,662)
Investments in life insurance contracts	3,354	4,073	6,967
Net cash provided (used) by investing activities	3,354	4,073	6,967
Long-term borrowings		650,000
Debt retirements	(19)	(561,394)	(10)
Debt issuance costs	(1,829)	(12,492)	(2,552)
Stock option exercises	2,023	1,462	893
Excess tax benefits from stock-based compensation			250
Common dividends paid	(2,091)	(8,354)	(8,328)
Net intercompany financing activities	18,477	23,391	95,708
Net cash provided (used) by financing activities	16,561	92,613	85,961
Increase (decrease) in cash and cash equivalents	(29,185)	41,406	55,266
Cash and cash equivalents at beginning of year	113,898	72,492	17,226
Cash and cash equivalents at end of year	84,713	113,898	72,492
Guarantor Subsidiaries
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	59,274	64,525	85,815
Capital expenditures-expansions	(72,906)	(25,430)	(5,337)
Capital expenditures-other	(33,430)	(20,253)	(8,322)
Proceeds from asset disposals	66,845	3,596	21,592
Other-net	(245)	1,266	2,079
Net cash provided (used) by investing activities	(39,736)	(40,821)	10,012
Debt retirements	(281)	(233)	(235)
Net intercompany financing activities	(18,477)	(23,391)	(95,708)
Net cash provided (used) by financing activities	(18,758)	(23,624)	(95,943)
Increase (decrease) in cash and cash equivalents	780	80	(116)
Cash and cash equivalents at beginning of year	2,534	2,454	2,570
Cash and cash equivalents at end of year	$ 3,314	$ 2,534	$ 2,454