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Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance [Line Items]
Unpaid losses	$ 2,077.6	$ 1,515.2	$ 560.7
Reinsurance Recoverables, Uncollateralized	1,497.8	1,001.9
Underwriting premiums	1,459.3	1,496.5
Due for settlement	12.4
Premiums Receivable, Allowance for Doubtful Accounts	$ 16.2	$ 5.2
Allowable holdings of a single issue or issuer, percentage	2.00%
Percentage of written premium major broker accounted minimum	10.00%
AM Best, A plus Rating | Standard & Poor's, A plus Rating | Everest Re
Reinsurance [Line Items]
Concentration Risk, Percentage		7.70%
AM Best, A plus Rating | Standard & Poor's, AA- Rating | Munich Re
Reinsurance [Line Items]
Concentration Risk, Percentage	15.70%	17.00%
AM Best, A plus Rating | Standard & Poor's, AA- Rating | Everest Re
Reinsurance [Line Items]
Concentration Risk, Percentage	10.20%
AM Best, A Rating | Standard & Poor's, A plus Rating | Lloyds
Reinsurance [Line Items]
Concentration Risk, Percentage	10.20%	13.80%

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- Definition

Percentage Of Allowable Holdings Of A Single Issuer Or Issuer, Minimum

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Percentage of written premium major broker accounted minimum.

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Premiums receivable non current.

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Reinsurance [Line Items]

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Reinsurance Recoverables, Uncollateralized

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- Definition

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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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 carrying amount as of the balance sheet date due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, net of the allowance for doubtful accounts.

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Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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