DERIVATIVE INSTRUMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	$ 5,859	$ 16,897
Derivative Liability Fair Value	13,717	15,576
Other Assets [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	5,859	16,897
Other Liabilities [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	13,717	15,576
Investment Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	147,015	195,979
Investment Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	12,331
Investment Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	439	87
Investment Portfolio [Member] \| Interest Rate Swap [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	180,000	0
Investment Portfolio [Member] \| Interest Rate Swap [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	393	0
Investment Portfolio [Member] \| Interest Rate Swap [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	0	0
Underwriting Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	479,818	492,899
Underwriting Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	5,466	2,034
Underwriting Portfolio [Member] \| Foreign Exchange Forward Contracts [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	1,434	8,989
Underwriting Portfolio [Member] \| Weather Related Derivative [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	0	67,957
Underwriting Portfolio [Member] \| Weather Related Derivative [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	2,532
Underwriting Portfolio [Member] \| Weather Related Derivative [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	0	6,500
Underwriting Portfolio [Member] \| Commodity Contract [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	0	0
Underwriting Portfolio [Member] \| Commodity Contract [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	0
Underwriting Portfolio [Member] \| Commodity Contract [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	0	0
Underwriting Portfolio [Member] \| Other Underwriting-related Contracts [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	85,000	0
Underwriting Portfolio [Member] \| Other Underwriting-related Contracts [Member] \| Other Assets [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	0
Underwriting Portfolio [Member] \| Other Underwriting-related Contracts [Member] \| Other Liabilities [Member] \| Not Designated as Hedging Instruments [Member]
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	$ 11,844	$ 0

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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