DERIVATIVE INSTRUMENTS - Balance sheet classification of derivatives recorded at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	$ 3,174	$ 8,237
Liability derivative fair value	13,637	14,522
Not Designated as Hedging Instruments \| Other assets
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Asset derivative fair value	3,174	8,237
Not Designated as Hedging Instruments \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	13,637	14,522
Not Designated as Hedging Instruments \| Investment Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	68,998	79,336
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	0	262
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	1,405	531
Not Designated as Hedging Instruments \| Investment Portfolio \| Interest rate swaps
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	0	150,000
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	0
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	0	1,116
Not Designated as Hedging Instruments \| Underwriting Portfolio \| Foreign exchange forward contracts
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Derivative notional amount	1,038,630	737,419
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	3,174	7,975
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	2,560	2,576
Not Designated as Hedging Instruments \| Underwriting Portfolio \| 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 \| 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 \| Other underwriting-related derivatives \| Other liabilities
Derivative Fair Values on Consolidated Balance Sheet [Line Items]
Liability derivative fair value	$ 9,672	$ 10,299

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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- Definition

Nominal or face amount used to calculate payment on derivative.

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