Pensions and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Defined Benefit Pension Plans [Member]
Components of net periodic benefit cost
Service cost	$ 2,320	$ 1,976	$ 4,618	$ 4,038
Interest cost	5,997	6,600	12,025	13,123
Expected return on plan assets	(6,327)	(6,714)	(12,649)	(13,599)
Recognized net actuarial loss (gain)	3,275	3,072	6,637	5,273
Settlement loss			6,669	0
Other	252	285	506	585
Net periodic benefit cost	5,517	5,219	17,806	9,420
Other Postretirement Benefits [Member]
Components of net periodic benefit cost
Service cost	559	545	1,232	1,070
Interest cost	781	772	1,546	1,552
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss (gain)	0	2	0	(23)
Settlement loss			0	0
Other	(106)	(113)	(212)	(213)
Net periodic benefit cost	$ 1,234	$ 1,206	$ 2,566	$ 2,386