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Marketable Securities - Additional Information (Detail) $ in Thousands | 12 Months Ended
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Jan. 31, 2017 USD ($) Securities | Jan. 31, 2016 USD ($) Securities | Jan. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sales and maturities of available-for-sale securities | $ 243,159 | $ 374,057 | $ 830,297
Amortization of discounts and premiums, net | $ 2,200 | $ 3,841 | 6,696
Total number of securities with unrealized loss positions within the Company's portfolio | Securities | 206 | 84 |
Other Income ( Expense ), Net |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Net realized gain (loss) | $ (83) | $ 43 | $ 237

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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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 2: 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=64930785&loc=d3e28541-108399 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=63488584&loc=d3e20235-122688 + Details
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