INVESTMENTS - Assets Measured At Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets measured at Fair Value
Available-for-sale securities	$ 1,913,534	$ 1,905,729
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	43,543	33,788
U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	15,740	6,747
Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale securities	4,478	10,665
Agency MBS
Assets measured at Fair Value
Available-for-sale securities	254,892	264,468
ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	91,948	135,304
Corporate
Assets measured at Fair Value
Available-for-sale securities	517,109	562,690
Municipal
Assets measured at Fair Value
Available-for-sale securities	610,400	481,425
Equity securities
Assets measured at Fair Value
Available-for-sale securities	375,424	410,642
Fair value measured on recurring basis
Assets measured at Fair Value
Available-for-sale securities	1,913,534	1,905,729
Fair value measured on recurring basis \| U.S. government
Assets measured at Fair Value
Available-for-sale securities	43,543	33,788
Fair value measured on recurring basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	15,740	6,747
Fair value measured on recurring basis \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale securities	4,478	10,665
Fair value measured on recurring basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale securities	254,892	264,468
Fair value measured on recurring basis \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	91,948	135,304
Fair value measured on recurring basis \| Corporate
Assets measured at Fair Value
Available-for-sale securities	517,109	562,690
Fair value measured on recurring basis \| Municipal
Assets measured at Fair Value
Available-for-sale securities	610,400	481,425
Fair value measured on recurring basis \| Equity securities
Assets measured at Fair Value
Available-for-sale securities	375,424	410,642
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	375,424	410,642
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	375,424	410,642
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale securities	1,538,110	1,495,087
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government
Assets measured at Fair Value
Available-for-sale securities	43,543	33,788
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency
Assets measured at Fair Value
Available-for-sale securities	15,740	6,747
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	4,478	10,665
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Agency MBS
Assets measured at Fair Value
Available-for-sale securities	254,892	264,468
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale securities	91,948	135,304
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate
Assets measured at Fair Value
Available-for-sale securities	517,109	562,690
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal
Assets measured at Fair Value
Available-for-sale securities	$ 610,400	$ 481,425

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=51813488&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=27724398&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=49167202&loc=d3e26610-111562

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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=36462937&loc=d3e19207-110258

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