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Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance [Line Items]
Unpaid losses	$ 1,515.2	$ 560.7	$ 354.8
Underwriting premiums	1,496.5	1,399.4
Due for settlement	8.7
Premiums Receivable, Allowance for Doubtful Accounts	$ 5.2	$ 5.0
Allowable holdings of a single issue or issuer, percentage	2.00%
Percentage of written premium major broker accounted minimum	10.00%
Markel
Reinsurance [Line Items]
Concentration Risk, Percentage	6.40%
Munich Re
Reinsurance [Line Items]
Concentration Risk, Percentage	5.50%
Lloyds
Reinsurance [Line Items]
Concentration Risk, Percentage	5.20%

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- Definition

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

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- Definition

Percentage of written premium major broker accounted minimum.

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- Definition

Premiums receivable non current.

+ References

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- Definition

Reinsurance [Line Items]

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

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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.

+ References

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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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