Long-Term Debt and Capital Leases (Components of Long Term Debt and Capital Leases) (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Jan. 31, 2013	Apr. 30, 2012
Senior Subordinated Notes Due 2019 Notes [Member]
Debt Instrument [Line Items]
Letters of credit due date	Feb. 15, 2019
Revenue Bonds Series 2005R-2 bearing interest	7.75%
Senior second lien notes, unamortized discount	$ 1,792	$ 0
Senior Secured Second Lien Notes, Due 2014 [Member]
Debt Instrument [Line Items]
Letters of credit due date	Jul. 15, 2014
Revenue Bonds Series 2005R-2 bearing interest	11.00%
Senior second lien notes, unamortized discount	0	3,536
Senior Secured Revolving Credit Facility, Due 2016 [Member]
Debt Instrument [Line Items]
Letters of credit	227,500
Letters of credit due date	Mar. 18, 2016
Interest at LIBOR	3.75%
One month LIBOR	3.96%
Revenue Bonds Series, 2005R-1, Due 2025 [Member]
Debt Instrument [Line Items]
Letters of credit due date	Jan. 01, 2025
One month LIBOR	0.14%
One month LIBOR	3.875%
Revenue Bonds Series 2005R-2, Due 2025 [Member]
Debt Instrument [Line Items]
Letters of credit due date	Jan. 01, 2025
Revenue Bonds Series 2005R-2 bearing interest	6.25%
Notes Payable [Member]
Debt Instrument [Line Items]
Capital leases interest rate	2.49%
Capital leases interest rate	6.50%
Capital leases installments	575
Capital Leases maturity date start	May 01, 2013
Capital Leases maturity date end	Apr. 30, 2017
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital leases interest rate	4.50%
Capital leases interest rate	4.72%
Capital leases installments	$ 78
Capital Leases maturity date start	Apr. 01, 2013
Capital Leases maturity date end	Jan. 31, 2015