FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 797,808	$ 770,923
Derivatives - Other underwriting-related derivatives	(22,245)	(13,637)
Fixed maturities	12,076,186	12,468,205
Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	37,441	36,934
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	50,234	51,262
Derivatives - Other underwriting-related derivatives	(20,164)	(9,672)
Fixed maturities	16,137	8,021
Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	37,441	$ 36,934
Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	$ (20,164)
Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.005
Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.005
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 12,793
Estimated maturity dates	7 years
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Estimated maturity dates	7 years
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Default rates
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.035
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Default rates | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.035
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Loss severity rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.350
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Loss severity rate | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.350
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Collateral spreads
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Other investments - CLO-Equities | Discounted cash flow | Significant unobservable inputs (Level 3) | Collateral spreads | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030