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Cash, Cash Equivalents and Short Term Investments - Additional Information (Detail) (USD $) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Apr. 30, 2015 | Jan. 31, 2015 | Apr. 30, 2014 | Jan. 31, 2014
Cash and Cash Equivalents [Line Items] |  |  |  |
Liquid investments, original maturities | Three months or less |  |  |
Short-term investments, maturity period | August to October 2015 |  |  |
Accrued interest on held-to-maturity securities | $ 17,000 agx_AccumulatedAccruedInterestOnHeldToMaturitySecurities |  |  |
Cash, cash equivalents | 274,528,000 us-gaap_CashAndCashEquivalentsAtCarryingValue | 333,691,000 us-gaap_CashAndCashEquivalentsAtCarryingValue | 313,761,000 us-gaap_CashAndCashEquivalentsAtCarryingValue | 272,209,000 us-gaap_CashAndCashEquivalentsAtCarryingValue
Short-term investments | 56,017,000 us-gaap_ShortTermInvestments |  |  |
Joint Venture [Member] |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Cash, cash equivalents | 105,300,000 us-gaap_CashAndCashEquivalentsAtCarryingValue / dei_LegalEntityAxis = us-gaap_CorporateJointVentureMember |  |  |
Short-term investments | $ 24,000,000 us-gaap_ShortTermInvestments / dei_LegalEntityAxis = us-gaap_CorporateJointVentureMember |  |  |

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- Definition Accumulated Accrued Interest On Held To Maturity Securities + References No definition available. + Details
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- Definition Original maturity period of highly liquid investments. + References No definition available. + Details
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- Definition Short Term Investments Maturity Period + References No definition available. + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3044-108585 + Details
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- Definition Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 + Details
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