Pretax Amounts in Accumulated Other Comprehensive Loss Expected to be Recognized as Components of Net Periodic Postretirement Benefit Cost (Credit) (Detail) (Defined Benefit Plan, USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2012	May 31, 2011
Pension Plans, Defined Benefit
Schedule of Pension and Other Postretirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss at beginning of year	$ 18,828	$ 21,021
Amortization of actuarial loss	(1,722)	(2,068)
Current period net actuarial loss (gain)	11,862	(125)
Net actuarial loss at the end of year	28,968	18,828
Other Postretirement Benefit Plans, Defined Benefit
Schedule of Pension and Other Postretirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss at beginning of year	5,613	6,382
Amortization of actuarial loss	(566)	(614)
Current period net actuarial loss (gain)	19	(155)
Net actuarial loss at the end of year	5,066	5,613
Net prior service credit at beginning of year	(4,320)	(5,095)
Amortization of prior service credit	775	775
Net prior service credit at the end of year	$ (3,545)	$ (4,320)