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INVESTMENTS - Equity securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | $ 398,956 | $ 365,905
Gross unrealized gains | 81,521 | 33,610
Gross unrealized losses | (6,270) | (17,882)
Fair value | $ 474,207 | 381,633
Limited partnership investment, other investments | 55.00% |
Common stocks |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | $ 504 | 790
Gross unrealized gains | 77 | 112
Gross unrealized losses | (388) | (375)
Fair value | 193 | 527
Exchange-traded funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | 215,986 | 213,420
Gross unrealized gains | 81,444 | 33,498
Gross unrealized losses | (105) | (10,079)
Fair value | 297,325 | 236,839
Bond mutual funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | 182,466 | 151,695
Gross unrealized gains | 0 | 0
Gross unrealized losses | (5,777) | (7,428)
Fair value | $ 176,689 | $ 144,267

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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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