Fair Value Measurements (Details 1) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Apr. 30, 2014
Total [Member]
Assets:
Assets measured at fair value	$ 352,500
Liabilities:
Liabilities measured at fair value	3,548
Level 1 [Member]
Assets:
Assets measured at fair value	226,925
Liabilities:
Liabilities measured at fair value	0
Level 2 [Member]
Assets:
Assets measured at fair value	125,385
Liabilities:
Liabilities measured at fair value	3,548
Level 3 [Member]
Assets:
Assets measured at fair value	190
Liabilities:
Liabilities measured at fair value	0
Cash and Cash Equivalents [Member] | Total [Member]
Assets:
Assets measured at fair value	246,780
Cash and Cash Equivalents [Member] | Level 1 [Member]
Assets:
Assets measured at fair value	226,782
Cash and Cash Equivalents [Member] | Level 2 [Member]
Assets:
Assets measured at fair value	19,998
Cash and Cash Equivalents [Member] | Level 3 [Member]
Assets:
Assets measured at fair value	0
Short-term Investments [Member] | Total [Member]
Assets:
Assets measured at fair value	90,049
Short-term Investments [Member] | Level 1 [Member]
Assets:
Assets measured at fair value	0
Short-term Investments [Member] | Level 2 [Member]
Assets:
Assets measured at fair value	90,049
Short-term Investments [Member] | Level 3 [Member]
Assets:
Assets measured at fair value	0
Prepaid Expenses and Other Current Assets [Member] | Total [Member]
Assets:
Assets measured at fair value	538
Prepaid Expenses and Other Current Assets [Member] | Level 1 [Member]
Assets:
Assets measured at fair value	52
Prepaid Expenses and Other Current Assets [Member] | Level 2 [Member]
Assets:
Assets measured at fair value	296
Prepaid Expenses and Other Current Assets [Member] | Level 3 [Member]
Assets:
Assets measured at fair value	190
Marketable Securities Noncurrent [Member] | Total [Member]
Assets:
Assets measured at fair value	15,042
Marketable Securities Noncurrent [Member] | Level 1 [Member]
Assets:
Assets measured at fair value	0
Marketable Securities Noncurrent [Member] | Level 2 [Member]
Assets:
Assets measured at fair value	15,042
Marketable Securities Noncurrent [Member] | Level 3 [Member]
Assets:
Assets measured at fair value	0
Other Noncurrent Assets [Member] | Total [Member]
Assets:
Assets measured at fair value	91
Other Noncurrent Assets [Member] | Level 1 [Member]
Assets:
Assets measured at fair value	91
Other Noncurrent Assets [Member] | Level 2 [Member]
Assets:
Assets measured at fair value	0
Other Noncurrent Assets [Member] | Level 3 [Member]
Assets:
Assets measured at fair value	0
Accrued Liabilities [Member] | Total [Member]
Liabilities:
Liabilities measured at fair value	3,548
Accrued Liabilities [Member] | Level 1 [Member]
Liabilities:
Liabilities measured at fair value	0
Accrued Liabilities [Member] | Level 2 [Member]
Liabilities:
Liabilities measured at fair value	3,548
Accrued Liabilities [Member] | Level 3 [Member]
Liabilities:
Liabilities measured at fair value	$ 0