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Securities (Details Textual) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |
Available-for-sale securities, gross realized gain (loss) | $ 0 |  | $ 0 | $ (8,000) |  |
Available-for-sale securities, gross realized losses |  | $ 0 |  | $ 0 |  |
Total fair value of investments | 450,578,000 |  | 450,578,000 |  |  | $ 346,357,000
Continuous unrealized loss position, fair value | $ 466,600,000 |  | $ 466,600,000 |  |  | $ 354,600,000
Continuous unrealized loss position, fair value (as a percent) |  |  | 96.00% |  | 72.00% |

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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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