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Fair Value Measurements - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Aug. 31, 2017 | May 31, 2017
--- | --- | ---
Fair value on a recurring basis |  |
Cash and cash equivalents | $ 191,414 | $ 169,266
Total assets at fair value | 213,040 | 191,485
Canadian treasury securities |  |
Fair value on a recurring basis |  |
Marketable securities | 21,626 | 22,219
Level 1 |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 191,414 | 169,266
Total assets at fair value | 191,414 | 169,266
Level 1 | Canadian treasury securities |  |
Fair value on a recurring basis |  |
Marketable securities | 0 | 0
Level 2 |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 0 | 0
Total assets at fair value | 21,626 | 22,219
Level 2 | Canadian treasury securities |  |
Fair value on a recurring basis |  |
Marketable securities | 21,626 | 22,219
Level 3 |  |
Fair value on a recurring basis |  |
Cash and cash equivalents | 0 | 0
Total assets at fair value | 0 | 0
Level 3 | Canadian treasury securities |  |
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=66048111&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=75030427&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=82897076&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=82896770&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 No definition available. + Details
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