FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 760,206	$ 770,923
Derivatives - Other underwriting-related derivatives	(11,026)	(13,637)
Fixed maturities	12,609,241	12,468,205
Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	39,493	36,934
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	48,682	51,262
Derivatives - Other underwriting-related derivatives	(9,448)	(9,672)
Fixed maturities	25,164	8,021
Significant unobservable inputs (Level 3) | Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	$ (9,448)
Significant unobservable inputs (Level 3) | Discounted cash flow | Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.005
Significant unobservable inputs (Level 3) | Discounted cash flow | Discount rate | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.005
Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 39,493	$ 36,934
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	$ 9,189
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	0.038
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	0.038
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