DERIVATIVE INSTRUMENTS (Schedule of Derivative Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	$ 3,468	$ 8,237
Derivative Liability Fair Value	13,837	14,522
Other assets \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Asset Fair Value	3,468	8,237
Other liabilities \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative Liability Fair Value	13,837	14,522
Investment Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	172,306	79,336
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	818	262
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	284	531
Investment Portfolio \| Interest rate swaps \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	85,000	150,000
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	984	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	0	1,116
Underwriting Portfolio \| Foreign exchange forward contracts \| Not Designated as Hedging Instruments
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	543,174	737,419
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	1,666	7,975
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	3,320	2,576
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	$ 10,233	$ 10,299

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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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 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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