Retirement Plans - Pretax Changes in Accumulated Other Comprehensive (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans, Net Actuarial (Gain) Loss [Roll Forward]
Net actuarial loss at beginning of year	$ 28,968	$ 18,828
Amortization of actuarial loss	(1,381)	(1,722)
Current period net actuarial loss (gain)	(6,467)	11,862
Net actuarial loss at the end of year	21,120	28,968
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans, Net Actuarial (Gain) Loss [Roll Forward]
Net actuarial loss at beginning of year	5,066	5,613
Amortization of actuarial loss	(515)	(566)
Current period net actuarial loss (gain)	(2,091)	19
Net actuarial loss at the end of year	2,460	5,066
Defined Benefit Plans and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) [Roll Forward]
Net prior service credit at beginning of year	(3,545)	(4,320)
Amortization of prior service credit	775	775
Net prior service credit at the end of year	$ (2,770)	$ (3,545)