Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - Ark $ in Millions	12 Months Ended
Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Gross	$ 368.2
Net	287.5
Ark
Segment Reporting Information [Line Items]
Ceded	$ (15.6)
Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	100.00%
Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 80.7
A+ or better
Segment Reporting Information [Line Items]
Gross	204.0
Net	$ 204.0
A+ or better \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	71.00%
A+ or better \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
A- to A
Segment Reporting Information [Line Items]
Gross	76.2
Net	$ 76.2
A- to A \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	26.50%
A- to A \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
B++ or lower and not rated
Segment Reporting Information [Line Items]
Gross	88.0
Net	$ 7.3
B++ or lower and not rated \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	2.50%
B++ or lower and not rated \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 80.7

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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