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INVESTMENTS (Equity Securities) (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Investment [Line Items] |  |
Cost | $ 376,007 | $ 552,867
Gross Unrealized Gains | 64,888 | 84,527
Gross Unrealized Losses | (7,584) | (1,883)
Fair Value | 433,311 | 635,511
Common stocks |  |
Investment [Line Items] |  |
Cost | 13,392 | 22,836
Gross Unrealized Gains | 1,345 | 3,412
Gross Unrealized Losses | (1,232) | (590)
Fair Value | 13,505 | 25,658
Exchange-traded funds |  |
Investment [Line Items] |  |
Cost | 211,940 | 356,252
Gross Unrealized Gains | 63,543 | 71,675
Gross Unrealized Losses | (2,035) | (294)
Fair Value | 273,448 | 427,633
Bond mutual funds |  |
Investment [Line Items] |  |
Cost | 150,675 | 173,779
Gross Unrealized Gains | 0 | 9,440
Gross Unrealized Losses | (4,317) | (999)
Fair Value | $ 146,358 | $ 182,220

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