DERIVATIVE INSTRUMENTS - Balance sheet classification of derivatives recorded at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative asset fair value	$ 18,875	$ 3,174
Derivative liability fair value	11,486	13,637
Not Designated as Hedging Instruments \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative asset fair value	18,875	3,174
Not Designated as Hedging Instruments \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative liability fair value	11,486	13,637
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	105,781	68,998
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative asset fair value	2	0
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative liability fair value	2,364	1,405
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	1,197,012	1,038,630
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative asset fair value	18,873	3,174
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative liability fair value	0	2,560
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Other underwriting-related contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	75,000	85,000
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Other underwriting-related contracts \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative asset fair value	0	0
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Other underwriting-related contracts \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative liability fair value	$ 9,122	$ 9,672

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.

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Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

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Nominal or face amount used to calculate payment on derivative.

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