Credit Facility (Details) (USD $)	Apr. 30, 2013
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, current borrowing capacity	$ 150,000,000
Letters of credit collateralized by the credit facility	$ 48,100,000
4.0% Convertible Senior Notes due March 15, 2015 [Member]
Line of Credit Facility [Line Items]
Interest rate on convertible notes	4.00%
4.0% Convertible Senior Notes due March 15, 2015 [Member] \| Convertible Notes Payable [Member]
Line of Credit Facility [Line Items]
Interest rate on convertible notes	4.00%

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant