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Available-for-Sale Debt Securities - Additional Information (Details) - position | Sep. 30, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Abstract] |  |
Securities in unrealized loss positions, number of positions (securities) | 1,523 | 2,957

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- Definition Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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 + Details
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