Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance [Line Items]
Unpaid losses	$ 3,195.2	$ 2,319.8	$ 2,077.6
Reinsurance Recoverables, Uncollateralized	2,010.0	1,817.5
Underwriting premiums (net of allowance for expected credit losses of $34.0 — $Nil)	1,279.8	1,318.4
Due for settlement	11.4
Premium Receivable, Allowance for Credit Loss	$ 34.0	23.0
Allowable holdings of a single issue or issuer, percentage	2.00%
Percentage of written premium major broker accounted minimum	10.00%
Reinsurance Recoverable, Allowance for Credit Loss	$ 3.8	$ 3.7
AM Best, A plus Rating \| Standard & Poor's, A plus Rating \| Everest Re
Reinsurance [Line Items]
Concentration risk percentage	13.40%	11.90%
AM Best, A plus Rating \| Standard & Poor's, AA- Rating \| Munich Re
Reinsurance [Line Items]
Concentration risk percentage	11.00%	15.10%
AM Best, A Rating \| Standard & Poor's, A plus Rating \| Lloyds
Reinsurance [Line Items]
Concentration risk percentage	9.20%	8.90%

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

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Percentage of written premium major broker accounted minimum.

+ References

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Premiums receivable non current.

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Reinsurance [Line Items]

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- Definition

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.

+ References

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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.

+ 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.

+ References

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- Definition

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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Amount of allowance for credit loss on reinsurance recoverable.

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