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Marketable Securities - Additional Information (Detail) (USD $) | 3 Months Ended | 6 Months Ended |  |  |  |  | 3 Months Ended |  |  |  |  | 3 Months Ended | 6 Months Ended
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Jul. 31, 2013 | Jul. 31, 2012 | Jul. 31, 2013 | Jul. 31, 2012 | Jan. 31, 2013 | Jul. 31, 2013 Long Term Investments | Jan. 31, 2013 Long Term Investments | Jul. 31, 2012 Long Term Investments | Apr. 30, 2013 Auction Rate Securities | Jan. 31, 2013 Auction Rate Securities | Jul. 31, 2012 Auction Rate Securities | Jan. 31, 2013 Auction Rate Securities Long Term Investments | Jul. 31, 2012 Auction Rate Securities Long Term Investments | Jul. 31, 2013 Other Expense Income Net | Jul. 31, 2012 Other Expense Income Net | Jul. 31, 2013 Other Expense Income Net | Jul. 31, 2012 Other Expense Income Net
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale and maturities of available-for-sale securities |  |  | $ 245,759,000 | $ 122,343,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Net realized gain (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,000 | 85,000 | (202,000) | 212,000
Amortization of discounts and premiums, net | 2,527,000 | 1,243,000 | 4,719,000 | 2,478,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of available-for-sale securities |  |  |  |  |  |  |  |  | 4,600,000 |  |  |  |  |  |  |  |
Amortized cost of available-for-sale securities | 442,558,000 | 227,773,000 | 442,558,000 | 227,773,000 | 378,393,000 | 216,833,000 | 150,107,000 | 92,058,000 |  |  |  | 4,925,000 | 4,925,000 |  |  |  |
Estimated fair value of available-for-sale securities | $ 442,565,000 | $ 227,554,000 | $ 442,565,000 | $ 227,554,000 | $ 378,071,000 | $ 216,766,000 | $ 149,585,000 | $ 91,664,000 |  | $ 4,330,000 | $ 4,330,000 | $ 4,330,000 | $ 4,330,000 |  |  |  |

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- Definition Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 5 + Details
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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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