FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 790,530	$ 829,156
Derivatives - Other underwriting-related derivatives	(28,167)	(11,486)
Fixed maturities, available for sale	11,728,611	12,041,799
Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	61,783	70,011
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	68,052	76,184
Derivatives - Other underwriting-related derivatives	(8,462)	(9,122)
Fixed maturities, available for sale	10,485	14,909
Significant unobservable inputs (Level 3) \| Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	61,783	$ 70,011
Significant unobservable inputs (Level 3) \| Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	$ (8,462)
Significant unobservable inputs (Level 3) \| Discounted cash flow \| Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.009
Significant unobservable inputs (Level 3) \| Discounted cash flow \| Discount rate \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.009
Significant unobservable inputs (Level 3) \| Valuation, Market Approach \| Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 40,000
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 6,269
Estimated maturity dates	5 years
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Estimated maturity dates	5 years
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Default rates
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.045
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Default rates \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.045
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Loss severity rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.500
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Loss severity rate \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.500
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Collateral spreads
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Significant unobservable inputs (Level 3) \| Other investments - CLO-Equities \| Discounted cash flow \| Collateral spreads \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 21,643
Estimated maturity dates	5 years
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Estimated maturity dates	5 years
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.013
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Discount rate \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.013
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Default rates
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.005
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Default rates \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.005
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Loss absorption yield
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.010
Significant unobservable inputs (Level 3) \| Other privately held investments \| Discounted cash flow \| Loss absorption yield \| Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.010

The estimated remaining time to maturity of the CLO structure (expressed as a number of years).

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- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

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Value of input used to measure alternative investment.

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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).

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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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.

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No definition available.

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