Derivative Financial Instruments and Fair Value Measurements - Fair Value Based on Quoted Market Prices (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2014	Apr. 30, 2013
Estimate of Fair Value, Fair Value Disclosure | Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 1,384,500	$ 1,391,000
Estimate of Fair Value, Fair Value Disclosure | Senior Unsecured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	313,500	0
Carrying (Reported) Amount, Fair Value Disclosure | Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	1,288,214	1,287,303
Carrying (Reported) Amount, Fair Value Disclosure | Senior Unsecured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 300,000	$ 0