INVESTMENTS - Assets Measured At Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets measured at Fair Value
Available-for-sale fixed income	$ 1,963,585	$ 1,983,086
Equity securities	356,403	460,630
Fair value measured on recurring basis
Assets measured at Fair Value
Available-for-sale fixed income	1,963,585	1,983,086
Equity securities	356,403	460,630
Other invested assets	12,056
Total assets at fair value	2,332,044	2,443,716
Fair value measured on recurring basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	186,974	193,661
Fair value measured on recurring basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	37,042	38,855
Fair value measured on recurring basis \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,316	7,628
Fair value measured on recurring basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	414,262	420,165
Fair value measured on recurring basis \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	223,601	224,870
Fair value measured on recurring basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	688,418	692,067
Fair value measured on recurring basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	405,972	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	356,403	460,630
Other invested assets	12,056
Total assets at fair value	368,459	460,630
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,958,782	1,981,316
Total assets at fair value	1,958,782	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	186,974	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	37,042	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	7,316	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	414,262	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	223,601	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	683,615	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	405,972	405,840
Fair value measured on recurring basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	4,803	1,770
Total assets at fair value	4,803	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	$ 4,803	$ 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
-Name Accounting Standards Codification
-Topic 820
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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
-Topic 326
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- 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
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- Definition

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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