DERIVATIVE INSTRUMENTS (Schedule of Derivative Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	$ 10,091	$ 5,125
Derivative Liability Fair Value	15,745	14,386
Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	10,091	5,125
Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	15,745	14,386
Investment Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	159,226	137,422
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	231	10
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	680	619
Investment Portfolio \| Interest rate swaps \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	166,000	191,000
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	0	448
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	975	1,556
Underwriting Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	963,373	698,959
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	9,860	4,667
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	3,501	701
Underwriting Portfolio \| Weather-related contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	0	0
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	0
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	0
Underwriting Portfolio \| Other underwriting-related contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Notional Amount	85,000	85,000
Underwriting Portfolio \| Other underwriting-related 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 \| Other underwriting-related contracts \| Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	$ 10,589	$ 11,510

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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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 825
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-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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