DERIVATIVE INSTRUMENTS - Balance sheet classification of derivatives recorded at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	$ 8,237	$ 5,125
Liability derivative fair value	14,522	14,386
Not Designated as Hedging Instruments \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	8,237	5,125
Not Designated as Hedging Instruments \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	14,522	14,386
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	79,336	137,422
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	262	10
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	531	619
Not Designated as Hedging Instruments \| Investment Portfolio \| Interest rate swaps
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	150,000	191,000
Not Designated as Hedging Instruments \| Investment Portfolio \| Interest rate swaps \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	0	448
Not Designated as Hedging Instruments \| Investment Portfolio \| Interest rate swaps \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	1,116	1,556
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	737,419	698,959
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	7,975	4,667
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	2,576	701
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	85,000	85,000
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	0	0
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Derivatives - Other underwriting-related derivatives \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	$ 10,299	$ 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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- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ 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
-Subparagraph (c)
-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
-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

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