INVESTMENTS - Assets Measured At Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets measured at Fair Value
Available-for-sale securities	$ 2,072,731	$ 1,974,428
Transfers in (out of) level 1	0	0
Transfers in (out of) level 2	0	0
U.S. government
Assets measured at Fair Value
Available-for-sale securities	91,689	76,563
U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	18,778	5,813
Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale securities	7,588	9,151
Mortgage-backed
Assets measured at Fair Value
Available-for-sale securities	328,471	284,069
ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	70,526	93,910
Corporate Debt
Assets measured at Fair Value
Available-for-sale securities	519,022	508,367
Municipal
Assets measured at Fair Value
Available-for-sale securities	636,165	627,336
Equity securities
Assets measured at Fair Value
Available-for-sale securities	400,492	369,219
Fair value measured on recurring basis
Assets measured at Fair Value
Available-for-sale securities	2,072,731	1,974,428
Fair value measured on recurring basis \| U.S. government
Assets measured at Fair Value
Available-for-sale securities	91,689	76,563
Fair value measured on recurring basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	18,778	5,813
Fair value measured on recurring basis \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale securities	7,588	9,151
Fair value measured on recurring basis \| Mortgage-backed
Assets measured at Fair Value
Available-for-sale securities	328,471	284,069
Fair value measured on recurring basis \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	70,526	93,910
Fair value measured on recurring basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale securities	519,022	508,367
Fair value measured on recurring basis \| Municipal
Assets measured at Fair Value
Available-for-sale securities	636,165	627,336
Fair value measured on recurring basis \| Equity securities
Assets measured at Fair Value
Available-for-sale securities	400,492	369,219
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Available-for-sale securities	400,492	369,219
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Assets measured at Fair Value
Available-for-sale securities	400,492	369,219
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale securities	1,672,239	1,605,209
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government
Assets measured at Fair Value
Available-for-sale securities	91,689	76,563
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	18,778	5,813
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale securities	7,588	9,151
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Mortgage-backed
Assets measured at Fair Value
Available-for-sale securities	328,471	284,069
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	70,526	93,910
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate Debt
Assets measured at Fair Value
Available-for-sale securities	519,022	508,367
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal
Assets measured at Fair Value
Available-for-sale securities	$ 636,165	$ 627,336

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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