Income Taxes - Components of Net Deferred Tax Liability (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2013	May 31, 2012
Income Tax Disclosure [Abstract]
Deferred compensation	$ 20,832	$ 22,626
Inventory costs	11,231	4,897
Accrued expenses not currently tax deductible	8,092	9,192
Goodwill	0	988
Pension and other postretirement benefits	7,493	11,042
Alternative minimum tax credit carryforward	28,808	28,808
Net operating loss carryforward	148,020	61,530
Other	7,412	5,998
Total deferred tax assets	231,888	145,081
Valuation allowance	(3,639)	(5,159)
Net deferred tax assets	228,249	139,922
Property, plant and equipment	196,220	103,583
Goodwill	457	0
Deferred real estate gains	20,044	19,049
Other	4,824	3,575
Total deferred tax liabilities	221,545	126,207
Net deferred tax asset	6,704	13,715
Less current deferred tax asset	18,774	10,713
Long-term deferred tax asset (liability)	$ (12,070)	$ 3,002