Fair Value of Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2015 investment
Fair Value, Assets Measured on Recurring and Nonrecurring Basis, Alternative [Abstract]
Less than Twelve Months, Fair Value	$ 192,847
Less than 12 Months, Aggregate Losses	(100)
Twelve Months or Longer, Fair Value	0
12 Months or Longer, Aggregate Losses	0
Fair Value	192,847
Aggregate Losses	(100)
Number of Positions	76
Commercial paper
Fair Value, Assets Measured on Recurring and Nonrecurring Basis, Alternative [Abstract]
Less than Twelve Months, Fair Value	33,353
Less than 12 Months, Aggregate Losses	(13)
Twelve Months or Longer, Fair Value	0
12 Months or Longer, Aggregate Losses	0
Fair Value	33,353
Aggregate Losses	(13)
U.S. agency securities
Fair Value, Assets Measured on Recurring and Nonrecurring Basis, Alternative [Abstract]
Less than Twelve Months, Fair Value	4,002
Less than 12 Months, Aggregate Losses	0
Twelve Months or Longer, Fair Value	0
12 Months or Longer, Aggregate Losses	0
Fair Value	4,002
Aggregate Losses	0
Corporate bonds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis, Alternative [Abstract]
Less than Twelve Months, Fair Value	150,498
Less than 12 Months, Aggregate Losses	(86)
Twelve Months or Longer, Fair Value	0
12 Months or Longer, Aggregate Losses	0
Fair Value	150,498
Aggregate Losses	(86)
U.S. government bonds
Fair Value, Assets Measured on Recurring and Nonrecurring Basis, Alternative [Abstract]
Less than Twelve Months, Fair Value	4,994
Less than 12 Months, Aggregate Losses	(1)
Twelve Months or Longer, Fair Value	0
12 Months or Longer, Aggregate Losses	0
Fair Value	4,994
Aggregate Losses	$ (1)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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