FAIR VALUE MEASUREMENTS - Level 3 fair value inputs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 1,009,373	$ 830,219
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 29,604
Default rates	3.80%
Loss severity rate	35.00%
Collateral spreads	3.00%
Estimated maturity dates (years)	7 years
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Discounted cash flow | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Default rates	3.80%
Loss severity rate	35.00%
Collateral spreads	3.00%
Estimated maturity dates (years)	7 years
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Liquidation value
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 1,809
Fair value of collateral	100.00%
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Liquidation value | Minimum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount Margin	0.10%
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Liquidation value | Maximum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount Margin	16.60%
Significant Unobservable Inputs (Level 3) | Other investments - CLO-Equities | Liquidation value | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value of collateral	100.00%
Discount Margin	2.80%
Significant Unobservable Inputs (Level 3) | Other privately held investments | Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 46,430
Significant Unobservable Inputs (Level 3) | Other privately held investments | Discounted cash flow | Minimum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	6.00%
Significant Unobservable Inputs (Level 3) | Other privately held investments | Discounted cash flow | Maximum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	8.50%
Significant Unobservable Inputs (Level 3) | Other privately held investments | Discounted cash flow | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	7.50%
Significant Unobservable Inputs (Level 3) | Derivatives - Other underwriting-related derivatives | Discounted cash flow
Fair Value Inputs Assets Quantitative Information [Line Items]
Net derivative assets	$ (11,510)
Discount rate	2.40%
Significant Unobservable Inputs (Level 3) | Derivatives - Other underwriting-related derivatives | Discounted cash flow | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	2.40%