Long-Term Debt (Narrative) (Details) (USD $)	12 Months Ended
	Apr. 30, 2011	Apr. 30, 2010
Acquisition obligations	$ 43,273,000	$ 28,701,000
Capital lease obligations	10,953,000	11,526,000
Capital lease obligation (in years)	12
Short-term credit facility	25,000,000
Payments to retire debt in 2012	3,400,000
Payments to retire debt in 2013	631,000,000
Payments to retire debt in 2014	1,600,000
Payments to retire debt in 2015	400,900,000
Payments to retire debt in 2016	2,700,000
Payments to retire debt beyond 2016	13,500,000
FHLB [Member]
Maximum funding by facility	276,100,000
Long-term debt due date	April 2012
Remaining funding available	251,100,000
Mortgage loans eligible as collateral to increase borrowing capacity	381,500,000
Interest rate	2.36%
Committed Line of Credit [Member]
Maximum funding by facility	1,700,000,000
Long-term debt due date	July 31, 2013
Minimum net worth required for compliance	650,000,000
Maximum amount for compliance of cleandown requirement	200,000,000
Net worth, current	1,400,000,000
Committed Line of Credit [Member] | Minimum [Member]
Facility bearing interest in excess of LIBOR	1.30%
Facility bearing interest in excess of PRIME	0.30%
Facility fee	0.20%
Committed Line of Credit [Member] | Maximum [Member]
Facility bearing interest in excess of LIBOR	2.80%
Facility bearing interest in excess of PRIME	1.80%
Facility fee	0.70%
Senior Notes 7.875 % Due January 2013 [Member]
Long-term debt due date	January 15, 2013
Stated interest rate	7.875%
Senior notes issued	600,000,000
Senior Notes 5.125 % Due October 2014 [Member]
Long-term debt due date	October 30, 2014
Stated interest rate	5.125%
Senior notes issued	400,000,000
Senior Note 6 3/4 Due in November 2004 [Member]
Credit facility repayment	250,000,000
$500 Million Facility [Member]
Credit facility repayment	$ 500,000,000
Minimum [Member]
Long-term debt due date	May 2011
Maximum [Member]
Long-term debt due date	December 2022