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MARKETABLE SECURITIES - Narrative (Details) | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2019 USD ($) Position | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) Position security | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($)
Investments Debt And Equity Securities [Abstract] |  |  |  |  |
Other than temporary impairment losses on debt securities related to credit losses | $ 0 |  | $ 0 |  | $ 0
Number of investment positions in an unrealized loss position | Position | 26 |  | 26 |  |
Number of securities other than agencies amount | security |  |  | 0 |  |
Fair value of securities pledged as collateral | $ 240,348,000 |  | $ 240,348,000 |  | $ 259,557,000
Number of held-to-maturity securities sold | 0 | 0 | 0 | 0 |

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