FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 779,200	$ 787,787
Derivatives - Other underwriting-related derivatives	(17,601)	(14,522)
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	46,173	65,789
Derivatives - Other underwriting-related derivatives	(10,136)	$ (10,299)
Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	$ (10,136)
Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.016
Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.016
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	$ 15,454
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
Other investments - Other privately held investments | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 30,719
Other investments - Other privately held investments | Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Other investments - Other privately held investments | Discounted cash flow | Significant unobservable inputs (Level 3) | Discount rate | Weighted average
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030