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Financial Instruments (Narrative) (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Oct. 31, 2018 | Oct. 31, 2017 | Oct. 31, 2018 | Oct. 31, 2017 | Jan. 31, 2018
Investments, All Other Investments [Abstract] |  |  |  |  |
Securities in a continuous unrealized loss position for more than 12 months | $ 0 |  | $ 0 |  | $ 0
Gain (loss) from sale or redemption of available for sale securities |  |  | 0 |  |
Proceeds from sale and maturity of marketable securities |  |  | 256,200,000 | $ 729,300,000 |
Equity securities | 112,300,000 |  | 112,300,000 |  | $ 112,300,000
Positive adjustment on certain of its privately held investments |  |  | 6,200,000 |  |
Other than temporary impairment losses, investments | $ 0 | $ 3,900,000 | $ 4,800,000 | $ 8,000,000.0 |

X
- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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X
- Definition Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References No definition available. + Details
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- Definition Amount of investment in equity security without readily determinable fair value. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714 + Details
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- Definition Amount of impairment loss on investment in equity security without readily determinable fair value. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714 + Details
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- Definition Amount of gain from upward price adjustment on investment in equity security without readily determinable fair value. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714 + Details
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- Definition The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period. + References No definition available. + Details
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