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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

X

- 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

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+ References

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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=116654751&loc=SL75117546-209714

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+ References

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-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

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Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

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