FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 802,064	$ 787,787
Derivatives - Other underwriting-related derivatives	(12,948)	(14,522)
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	46,250	65,789
Derivatives - Other underwriting-related derivatives	(10,262)	$ (10,299)
Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	(10,262)
Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 17,798
Estimated maturity dates	7 years
Discounted cash flow | Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 28,452
Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Estimated maturity dates	7 years
Default rates | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Default rates | Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Loss severity rate | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.350
Loss severity rate | Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.350
Collateral spreads | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Collateral spreads | Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Discount rate | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.019
Discount rate | Discounted cash flow | Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030
Discount rate | Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.019
Discount rate | Weighted average | Discounted cash flow | Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable input	0.030