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INVESTMENTS (Equity Securities) (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | $ 510,891 | $ 421,744
Gross unrealized gains | 109,534 | 97,648
Gross unrealized losses | (1,603) | (947)
Fair value | $ 618,822 | 518,445
Limited partnership |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Percentage of investment composition (as a percent) | 72.00% |
Common stocks |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | $ 943 | 10,810
Gross unrealized gains | 493 | 689
Gross unrealized losses | (409) | (557)
Fair value | 1,027 | 10,942
Preferred shares |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | 115 | 6,301
Gross unrealized gains | 148 | 1,767
Gross unrealized losses | 0 | 0
Fair value | 263 | 8,068
Exchange-traded funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | 203,336 | 147,794
Gross unrealized gains | 107,163 | 74,314
Gross unrealized losses | (659) | (390)
Fair value | 309,840 | 221,718
Bond mutual funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost | 306,497 | 256,839
Gross unrealized gains | 1,730 | 20,878
Gross unrealized losses | (535) | 0
Fair value | $ 307,692 | $ 277,717

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