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Investments - Narrative (Details)	Jun. 30, 2018	Dec. 31, 2017
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Fair value of investments, unrealized loss position	83.00%	78.00%

X

- Definition

Available for sale securities unrealized loss position percentage of total fair value.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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Period Type: