FAIR VALUE MEASUREMENTS - Level 3 fair value inputs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value	$ 787,787	$ 1,009,373
Net derivative liability	(14,522)	(14,386)
Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value	65,789	77,843
Net derivative liability	(10,299)	$ (11,510)
Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value	$ 21,271
Estimated maturity dates (years)	7 years
Significant unobservable inputs (Level 3) | Other investments - CLO-Equities | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Estimated maturity dates (years)	7 years
Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value	$ 44,518
Default rates | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.030
Default rates | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.030
Loss severity rate | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.35
Loss severity rate | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.35
Collateral spreads | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.030
Collateral spreads | Significant unobservable inputs (Level 3) | Other investments - CLO-Equities | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.030
Discount rate | Significant unobservable inputs (Level 3)
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivative liability	0.026
Discount rate | Significant unobservable inputs (Level 3) | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Derivative liability	0.026
Discount rate | Significant unobservable inputs (Level 3) | Other privately held investments | Minimum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.030
Discount rate | Significant unobservable inputs (Level 3) | Other privately held investments | Maximum
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.080
Discount rate | Significant unobservable inputs (Level 3) | Other privately held investments | Weighted Average
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	0.071