INVESTMENTS - Equity securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Cost	$ 365,905	$ 552,867
Gross unrealized gains	33,610	84,527
Gross unrealized losses	(17,882)	(1,883)
Fair value	381,633	635,511
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Cost	790	22,836
Gross unrealized gains	112	3,412
Gross unrealized losses	(375)	(590)
Fair value	527	25,658
Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Cost	213,420	356,252
Gross unrealized gains	33,498	71,675
Gross unrealized losses	(10,079)	(294)
Fair value	236,839	427,633
Bond mutual funds
Debt and Equity Securities, FV-NI [Line Items]
Cost	151,695	173,779
Gross unrealized gains	0	9,440
Gross unrealized losses	(7,428)	(999)
Fair value	$ 144,267	$ 182,220

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of unrealized gain on 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 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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