Pensions and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2011	Oct. 31, 2010
Defined Benefit Pension Plans [Member]
Net periodic benefit cost
Service cost	$ 1,976	$ 1,861	$ 4,038	$ 3,719
Interest cost	6,600	6,360	13,123	12,706
Expected return on plan assets	(6,714)	(6,674)	(13,599)	(13,331)
Recognized net actuarial loss (gain)	3,072	2,198	5,273	3,925
Termination benefit cost	0	735	0	8,197
Curtailment	0	181	0	4,091
Other	285	289	585	577
Net periodic benefit cost	5,219	4,950	9,420	19,884
Other Postretirement Benefits [Member]
Net periodic benefit cost
Service cost	545	405	1,070	810
Interest cost	772	691	1,552	1,381
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss (gain)	2	(134)	(23)	(268)
Termination benefit cost	0	0	0	2,413
Curtailment	0	0	0	0
Other	(113)	(122)	(213)	(244)
Net periodic benefit cost	$ 1,206	$ 840	$ 2,386	$ 4,092