FAIR VALUE MEASUREMENTS (Significant Unobservable Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 1,009,587	$ 1,009,373
Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Derivatives - Other underwriting-related derivatives
Fair Value Inputs Assets Quantitative Information [Line Items]
Net derivative assets (liabilities)	$ (10,942)
Discount rate	2.70%
Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 27,118
Default rates	3.00%
Loss severity rate	35.00%
Collateral spreads	3.00%
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	$ 48,787
Liquidation value \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Other investments	$ 1,438
Fair value of collateral	100.00%
Minimum \| Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	3.00%
Minimum \| Liquidation value \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount margin	0.10%
Maximum \| Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	8.50%
Maximum \| Liquidation value \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount margin	12.00%
Weighted Average \| Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Derivatives - Other underwriting-related derivatives
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	2.70%
Weighted Average \| Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Default rates	3.00%
Loss severity rate	35.00%
Collateral spreads	3.00%
Estimated maturity dates	7 years
Weighted Average \| Discounted cash flow \| Significant Unobservable Inputs (Level 3) \| Other privately held investments
Fair Value Inputs Assets Quantitative Information [Line Items]
Discount rate	7.20%
Weighted Average \| Liquidation value \| Significant Unobservable Inputs (Level 3) \| Other investments - CLO-Equities
Fair Value Inputs Assets Quantitative Information [Line Items]
Fair value of collateral	100.00%
Discount margin	2.10%

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

No definition available.

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- Definition

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.

+ Details

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- Definition

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

+ References

No definition available.

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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 of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

+ 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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- Definition

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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- Definition

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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- Definition

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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