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Components of marketable securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
May 31, 2021 | May 31, 2020
Marketable Securities [Line Items] |  |
Amortized Cost | $ 305,279 | $ 276,850
Unrealized gains | 239 | 577
Unrealized Losses | (33) | (23)
Fair Value | 305,485 | 277,404
US Treasury Securities [Member] |  |
Marketable Securities [Line Items] |  |
Amortized Cost | 0 | 2,502
Unrealized gains | 0 | 30
Unrealized Losses | 0 | 0
Fair Value | 0 | 2,532
Commercial PaperAnd Corporate Bonds [Member] |  |
Marketable Securities [Line Items] |  |
Amortized Cost | 299,524 | 257,700
Unrealized gains | 209 | 347
Unrealized Losses | (33) | (23)
Fair Value | 299,700 | 258,024
Certificates of Deposit [Member] |  |
Marketable Securities [Line Items] |  |
Amortized Cost | 5,755 | 16,648
Unrealized gains | 30 | 200
Unrealized Losses | 0 | 0
Fair Value | $ 5,785 | $ 16,848

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