Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Feb. 28, 2014	Feb. 28, 2013
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	$ (665)	$ 9,771
Capital expenditures – expansions	(7,125)	(61,344)
Capital expenditures – other	(26,152)	(17,752)
Proceeds from asset disposals	8,390	5,783
Proceeds from insurance contracts, net of premiums paid	4,386	2,366
Other – net	0	(67)
Net cash provided (used) by investing activities	(20,501)	(71,014)
Debt payments	(1,373)	(2,236)
Stock option exercises	9,076	7,116
Net intercompany financing activities	0	0
Net cash used by financing activities	7,703	4,880
Decrease in cash and cash equivalents	(13,463)	(56,363)
Cash and cash equivalents at beginning of period	61,296	88,027
Cash and cash equivalents at end of period	47,833	31,664
Parent Company
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	(71,557)	(88,158)
Capital expenditures – expansions	0	0
Capital expenditures – other	0	0
Proceeds from asset disposals	0	0
Proceeds from insurance contracts, net of premiums paid	4,386	2,366
Other – net		0
Net cash provided (used) by investing activities	4,386	2,366
Debt payments	0	(4)
Stock option exercises	9,076	7,116
Net intercompany financing activities	45,261	20,382
Net cash used by financing activities	54,337	27,494
Decrease in cash and cash equivalents	(12,834)	(58,298)
Cash and cash equivalents at beginning of period	58,791	84,713
Cash and cash equivalents at end of period	45,957	26,415
Guarantor Subsidiaries
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	70,892	97,929
Capital expenditures – expansions	(7,125)	(61,344)
Capital expenditures – other	(26,152)	(17,752)
Proceeds from asset disposals	8,390	5,783
Proceeds from insurance contracts, net of premiums paid	0	0
Other – net		(67)
Net cash provided (used) by investing activities	(24,887)	(73,380)
Debt payments	(1,373)	(2,232)
Stock option exercises	0	0
Net intercompany financing activities	(45,261)	(20,382)
Net cash used by financing activities	(46,634)	(22,614)
Decrease in cash and cash equivalents	(629)	1,935
Cash and cash equivalents at beginning of period	2,505	3,314
Cash and cash equivalents at end of period	1,876	5,249
Consolidation, Eliminations
Cash Flow Supplemental Disclosures [Line Items]
Net cash provided (used) by operating activities	0	0
Capital expenditures – expansions	0	0
Capital expenditures – other	0	0
Proceeds from asset disposals	0	0
Proceeds from insurance contracts, net of premiums paid	0	0
Other – net		0
Net cash provided (used) by investing activities	0	0
Debt payments	0	0
Stock option exercises	0	0
Net intercompany financing activities	0	0
Net cash used by financing activities	0	0
Decrease in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	$ 0	$ 0