FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 916,191	$ 1,009,373
Derivatives - Other underwriting-related derivatives	(15,745)	$ (14,386)
Significant Unobservable Inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivatives - Other underwriting-related derivatives	(10,589)
Significant Unobservable Inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	47,613
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	$ 24,715
Estimated maturity dates	7 years
Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments, at fair value	$ 1,438
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
Fair value of collateral | Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	1.00
Fair value of collateral | Weighted Average | Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	1.00
Discount margin | Minimum | Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.000
Discount margin | Maximum | Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.109
Discount margin | Weighted Average | Liquidation value | Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.022
Discount rate | Significant Unobservable Inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.029
Discount rate | Minimum | Significant Unobservable Inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.030
Discount rate | Maximum | Significant Unobservable Inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.085
Discount rate | Weighted Average | Significant Unobservable Inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.029
Discount rate | Weighted Average | Significant Unobservable Inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Unobservable Input	0.072