Schedule IV - Reinsurance Premiums Written and Premiums Earned (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
Direct	$ 2,446.6	$ 2,531.7	$ 2,341.4
Assumed	1,521.0	1,807.0	1,597.0
Ceded	(1,385.7)	(1,442.7)	(1,350.7)
Net written premiums	2,581.9	2,896.0	2,587.7
Gross Amount	2,444.8	2,370.8	2,139.1
Assumed From Other Companies	1,562.0	1,617.2	1,479.2
Ceded	(1,392.3)	(1,299.3)	(1,207.8)
Net premiums earned	$ 2,614.5	$ 2,688.7	$ 2,410.5
Percentage of Amount Assumed to Net	59.70%	60.10%	61.40%
Gross written premiums	$ 3,967.6	$ 4,338.7	$ 3,938.4

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Amount of earned premiums assumed from other entities.

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