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Investments - Narrative (Details) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) investment | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) investment | Jun. 30, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Investment impairment charges | $ | $ 121 | $ 16 | $ 374 | $ 67
Number of securities in gross unrealized loss position | investment | 28 |  | 28 |
Number of securities in gross unrealized loss position for longer than 12 months | investment | 8 |  | 8 |
Total unrealized loss | $ | $ 91 |  | $ 91 |

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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 + Details
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- Definition Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562 + Details
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