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COMMITMENTS AND CONTINGENCIES - Reinsurance and premiums (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) broker | Dec. 31, 2016 USD ($) | Dec. 31, 2015
Concentration Risk [Line Items] |  |  |
Premiums receivable allowance for doubtful accounts | $ 6,000,000 | $ 2,000,000 |
Premiums receivable, bad debt expense charges | $ 0 | $ 1,000,000 |
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 | 11.00% | 10.00% |
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Reinsurer 3 |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 7.00% | 9.00% |
Reinsurance Recoverable On Unpaid And Paid Losses | Reinsurer Concentration | Top Ten Reinsurers | AM Best, A plus Rating |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 56.00% | 67.00% |
Gross Premiums Written | Customer Concentration |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 49.00% | 52.00% | 53.00%
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 | 20.00% | 21.00% | 22.00%
Gross Premiums Written | Customer Concentration | Aon plc |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 17.00% | 19.00% | 18.00%
Gross Premiums Written | Customer Concentration | Willis Towers Watson PLC |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage of total | 12.00% | 12.00% | 13.00%

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