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Fair Value Measurements - Financial Instruments on a Recurring Basis (Details) - USD ($) $ in Thousands | Nov. 30, 2015 | May. 31, 2015
--- | --- | ---
Fair value on a recurring basis |  |
Marketable securities | $ 65,826 | $ 16,081
Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 606,785 | 417,073
Total assets at fair value | 672,611 | 433,154
Level 1 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 556,809 | 417,073
Total assets at fair value | 556,809 | 417,073
Level 2 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 49,976 | 0
Total assets at fair value | 115,802 | 16,081
Level 3 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 0 | 0
Total assets at fair value | 0 | 0
Canadian treasury securities | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Marketable securities | 65,826 | 16,081
Canadian treasury securities | Level 1 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Marketable securities | 0 | 0
Canadian treasury securities | Level 2 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Marketable securities | 65,826 | 16,081
Canadian treasury securities | Level 3 | Fair Value, Measurements, Recurring |  |
Fair value on a recurring basis |  |
Marketable securities | $ 0 | $ 0

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + 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=51813488&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=49167202&loc=d3e26610-111562 + Details
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- Definition Fair value portion 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. + 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 + Details
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