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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

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The amount of decrease during the period to the valuation allowance account representing portions of such account associated with premium receivables that have been determined to be absolutely uncollectible or that were sold or transferred, and which receivables were simultaneously written off.

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COMMITMENTS AND CONTINGENCIES - Reinsurance and premiums (Details)	12 Months Ended
	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%