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Fair Value - Additional Information (Detail) (USD $) |  |  |  | 3 Months Ended |  |  |  |  | 3 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 31, 2013 | Jan. 31, 2013 | Jul. 31, 2012 | Apr. 30, 2013 Auction Rate Securities | Jan. 31, 2013 Auction Rate Securities | Jul. 31, 2012 Auction Rate Securities | Jan. 31, 2013 Fair Value, Inputs, Level 3 | Jul. 31, 2012 Fair Value, Inputs, Level 3 | Jul. 31, 2013 Fair Value, Inputs, Level 3 Auction Rate Securities | Jan. 31, 2013 Fair Value, Inputs, Level 3 Auction Rate Securities | Jul. 31, 2012 Fair Value, Inputs, Level 3 Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of available-for-sale securities |  |  |  | $ 4,600,000 |  |  |  |  | $ 4,600,000 |  |
Amortized cost of available-for-sale securities | 442,558,000 | 378,393,000 | 227,773,000 |  |  |  |  |  |  | 4,925,000,000 | 4,925,000,000
Estimated fair value of available-for-sale securities | $ 442,565,000 | $ 378,071,000 | $ 227,554,000 |  | $ 4,330,000 | $ 4,330,000 | $ 4,330,000 | $ 4,330,000 |  | $ 4,330,000 | $ 4,330,000

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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