Reconciliation of Changes in Projected Benefit Obligation and Fair Value of Plan Assets and Funded Status for Gerstenslager Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Change in benefit obligation
Benefit obligation, beginning of year	$ 35,539	$ 31,930
Interest cost	1,541	1,403	$ 1,361
Actuarial loss	3,924	3,002
Benefits paid	(777)	(796)
Benefit obligation, end of year	40,227	35,539	31,930
Change in plan assets
Fair value, end of year	28,160	26,470	24,442
Fair value, beginning of year	26,470	24,442
Actual return on plan assets	1,846	2,524	$ 4,355
Company contributions	620	300
Benefits paid	(777)	(796)
Funded status	(12,068)	(9,069)
Amounts recognized in the consolidated balance sheets consist of:
Other liabilities	(12,067)	(9,069)
Amounts recognized in accumulated other comprehensive loss consist of:
Net loss	17,900	14,028
Defined Benefit Plan
Change in plan assets
Fair value, end of year	28,159	26,470
Fair value, beginning of year	26,470
Other Postretirement Benefit Plan
Amounts recognized in the consolidated balance sheets consist of:
Accumulated other comprehensive loss	17,900	14,028
Amounts recognized in accumulated other comprehensive loss consist of:
Total	$ 17,900	$ 14,028