INVESTMENTS - Assets Measured At Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,075,093	$ 1,983,086
Equity securities	422,198	460,630
Fair value measured on recurring basis
Assets measured at Fair Value
Available-for-sale fixed income	2,075,093	1,983,086
Equity securities	422,198	460,630
Other invested assets	15,792
Total assets at fair value	2,513,083	2,443,716
Fair value measured on recurring basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	186,838	193,661
Fair value measured on recurring basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	32,091	38,855
Fair value measured on recurring basis \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,692	7,628
Fair value measured on recurring basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	409,167	420,165
Fair value measured on recurring basis \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	218,507	224,870
Fair value measured on recurring basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	765,125	692,067
Fair value measured on recurring basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	455,673	405,840
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	422,198	460,630
Other invested assets	15,792
Total assets at fair value	437,990	460,630
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,068,034	1,981,316
Total assets at fair value	2,068,034	1,981,316
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	186,838	193,661
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	32,091	38,855
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 fixed income	7,692	7,628
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	409,167	420,165
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	218,507	224,870
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	758,066	690,297
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	455,673	405,840
Fair value measured on recurring basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	7,059	1,770
Total assets at fair value	7,059	1,770
Fair value measured on recurring basis \| Significant Unobservable Inputs (Level 3) \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	$ 7,059	$ 1,770

Fair value of other invested assets.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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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
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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.

+ References

No definition available.

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