DERIVATIVE INSTRUMENTS - Balance sheet classification of derivatives recorded at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	$ 5,125	$ 16,897
Derivative Liability Fair Value	14,386	15,576
Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	5,125	16,897
Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	14,386	15,576
Investment Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	137,422	195,979
Investment Portfolio \| Foreign exchange forward contracts \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	10	12,331
Investment Portfolio \| Foreign exchange forward contracts \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	619	87
Investment Portfolio \| Interest rate swaps \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	191,000	0
Investment Portfolio \| Interest rate swaps \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	448	0
Investment Portfolio \| Interest rate swaps \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	1,556	0
Underwriting Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	698,959	492,899
Underwriting Portfolio \| Foreign exchange forward contracts \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	4,667	2,034
Underwriting Portfolio \| Foreign exchange forward contracts \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	701	8,989
Underwriting Portfolio \| Weather-related contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	0	67,957
Underwriting Portfolio \| Weather-related contracts \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	2,532
Underwriting Portfolio \| Weather-related contracts \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	0	6,500
Underwriting Portfolio \| Commodity contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	0	0
Underwriting Portfolio \| Commodity contracts \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	0
Underwriting Portfolio \| Commodity contracts \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	0	0
Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	85,000	0
Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives \| Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	0	0
Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	$ 11,510	$ 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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