Investment Securities Fair Value and Gross Unrealized Losses for Investments in a Gross Loss Position (Details) $ in Thousands	May 31, 2018 USD ($)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract] (Deprecated 2018-01-31)
Less than 12 months, Fair value, held-to-maturity	$ 308,602
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4,813
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	308,602
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	4,813
Corporate debt securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract] (Deprecated 2018-01-31)
Less than 12 months, Fair value, held-to-maturity	280,139
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4,595
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	280,139
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	4,595
Non-agency
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract] (Deprecated 2018-01-31)
Less than 12 months, Fair value, held-to-maturity	1,451
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	1,451
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	3
Asset-backed Securities
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract] (Deprecated 2018-01-31)
Less than 12 months, Fair value, held-to-maturity	27,012
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	215
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	27,012
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	$ 215

No definition available.

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

+ 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 (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

+ 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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

+ References

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