FAIR VALUE MEASUREMENTS (Details 2) $ in Thousands	6 Months Ended
Jun. 30, 2017 USD ($) degree_day	Dec. 31, 2016 USD ($)
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 813,617	$ 830,219
Discounted cash flow [Member] \| Level 3 [Member] \| Other Underwriting-related Derivatives [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net derivative assets	$ (12,135)
Discount rate	2.50%
Discounted cash flow [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 33,266
Default rates	3.80%
Loss severity rate	35.00%
Collateral spreads	3.00%
Estimated maturity dates	7 years
Discounted cash flow [Member] \| Level 3 [Member] \| Other Privately Held Investments [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 42,938
Liquidation Value Valuation Technique [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 13,810
Fair value of collateral	100.00%
Simulation model [Member] \| Level 3 [Member] \| Weather Related Derivative [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net derivative assets	$ (74)
Weather curve \| degree_day	4,526
Weather standard deviation \| degree_day	360
Minimum [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Other Privately Held Investments [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	6.00%
Minimum [Member] \| Liquidation Value Valuation Technique [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount margin	0.00%
Maximum [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Other Privately Held Investments [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	8.00%
Maximum [Member] \| Liquidation Value Valuation Technique [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount margin	16.20%
Weighted Average [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Other Underwriting-related Derivatives [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	2.50%
Weighted Average [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Default rates	3.80%
Loss severity rate	35.00%
Collateral spreads	3.00%
Estimated maturity dates	7 years
Weighted Average [Member] \| Discounted cash flow [Member] \| Level 3 [Member] \| Other Privately Held Investments [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	7.50%
Weighted Average [Member] \| Liquidation Value Valuation Technique [Member] \| Level 3 [Member] \| Other Investments - CLO-Equities [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value of collateral	100.00%
Discount margin	2.00%

Percentage of fair value of the underlying collateral, used as an input to measure fair value.

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The difference between the interest yield earned on the underlying collateral assets and the interest rates incurred on the debt tranches of the CLO structure.

+ References

No definition available.

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Discount margin applied to underlying collateral, used as an input to measure fair value.

+ References

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- Definition

The estimated remaining time to maturity of the CLO structure (expressed as a number of years).

+ References

No definition available.

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- Definition

Fair value inputs, heating degree days.

+ References

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Fair value inputs, standard deviation of heating degree days.

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- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

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- Definition

Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

+ References

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