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COMMITMENTS AND CONTINGENCIES - Reinsurance and premiums (Details) | 12 Months Ended
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Dec. 31, 2019 USD ($) broker | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Concentration Risk [Line Items] |  |  |
Premiums receivable allowance for doubtful accounts | $ 7,000,000 | $ 4,000,000 |
Premiums receivable, bad debt expense charges | $ 0 | $ 0 | $ 0
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Reinsurer 1 |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 12.00% | 13.00% |
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Reinsurer 2 |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 10.00% | 10.00% |
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Reinsurer 3 |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 9.00% | 10.00% |
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Top Ten Reinsurers | AM Best, A- Rating |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 89.10% | 89.50% |
Gross Premiums Written | Customer Concentration |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 47.00% | 43.00% | 49.00%
Concentration risk, number of brokers exceeding threshold | broker | 3 |  |
Gross Premiums Written | Customer Concentration | Marsh & McLennan Companies Inc. |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 18.00% | 17.00% | 20.00%
Gross Premiums Written | Customer Concentration | Aon plc |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 19.00% | 17.00% | 17.00%
Gross Premiums Written | Customer Concentration | Willis Towers Watson PLC |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 10.00% | 9.00% | 12.00%

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