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Investments - Additional Information (Detail) (USD $)
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3 Months Ended | 9 Months Ended | ||
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Feb. 28, 2014
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Feb. 28, 2013
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Feb. 28, 2014
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Feb. 28, 2013
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| Summary of Investment Holdings [Line Items] | ||||
| Proceeds on sales/maturities of investments | $ 0 | $ 5,500,000.0 | $ 19,000,000 | $ 5,500,000 |
| Investment purchases | 200,000 | 19,800,000 | 6,400,000 | |
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Minimum [Member]
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| Summary of Investment Holdings [Line Items] | ||||
| Maturity dates of new bonds | 9 years | |||
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Maximum [Member]
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| Summary of Investment Holdings [Line Items] | ||||
| Maturity dates of new bonds | 28 years | |||
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Greek bonds [Member]
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| Summary of Investment Holdings [Line Items] | ||||
| Face value of bonds | $ 11,700,000 | $ 11,700,000 | ||