| **Condensed consolidating guarantor and non-guarantor financial information (Narrative) (Details) (Manufacturing Operations [Member])** | Apr. 30, 2012 |
|-----------------------------------------------------------------------------------------------------------------------------------------|---------------|
| Senior Notes [Member]                                                                                                                   |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                               |          |
| Debt Instrument, Interest Rate, Stated Percentage                                                                                       | 8.25%         |
| Tax Exempt Bond [Member]                                                                                                                |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                               |          |
| Debt Instrument, Interest Rate, Stated Percentage                                                                                       | 6.50%         |

| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CondensedFinancialStatementsCaptionsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest rate stated in the contractual debt agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_DebtInstrumentInterestRateStatedPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |