FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 865,238	$ 829,156
Derivatives - Other underwriting-related derivatives	(18,382)	(11,486)
Fixed maturities, available for sale	11,898,300	12,041,799
Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	64,786	70,011
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	71,788	76,184
Derivatives - Other underwriting-related derivatives	(8,117)	(9,122)
Fixed maturities, available for sale	35,286	14,909
Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	64,786	$ 70,011
Significant unobservable inputs (Level 3) | Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	$ (8,117)
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	$ 43,000
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	$ 7,002
Estimated maturity dates	7 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	7 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]
Unobservable input	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]
Unobservable input	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