Investment Securities Additional Information (Details) - USD ($)	12 Months Ended
May 31, 2018	May 31, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale, Maturity and Collection of Investments	$ 0	$ 0
Debt and Equity Securities, Realized Gain (Loss)	0	0
Debt Securities, Held-to-maturity	$ 519,519,000	$ 0
Held-to-maturity security in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	225
Average Contractual Maturity of Held-to-maturity Securities	4 years
Held To Maturity Securities Expected Debt Maturities Weighted Average Yield	2.91%

Average Contractual Maturity of Held-to-maturity Securities

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- Definition

Held To Maturity Securities Expected Debt Maturities Weighted Average Yield

+ References

No definition available.

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- Definition

Amount of realized gain (loss) on investment in debt and equity securities.

+ References

No definition available.

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- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- References

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- Definition

The cash inflow associated with the sale, maturity and collection of all investments such as debt, security and so forth during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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