Fair Value Measurements and Financial Instruments (Details) (USD $)	Apr. 30, 2011
Fair Value Measurements and Financial Instruments
Fair value of 3.0% convertible senior notes	$ 212,500,000
Amount invested in money market mutual funds disclosed at fair value.	$ 205,815,000

- Definition

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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