Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earned premiums:
Net earned premiums	$ 353.2	$ 220.1	$ 783.8	$ 455.4
Ark
Written premiums:
Gross	215.5	162.4	1,252.5	895.0
Ceded	(22.9)	(41.5)	(246.0)	(169.5)
Net written premiums	192.6	120.9	1,006.5	725.5
Earned premiums:
Gross	454.9	284.5	966.6	622.0
Ceded	(108.8)	(71.1)	(208.8)	(186.2)
Net earned premiums	346.1	213.4	757.8	435.8
Losses and loss adjustment expenses:
Gross	391.4	193.5	712.9	372.7
Ceded	(177.7)	(64.3)	(256.7)	(124.9)
Net losses and loss adjustment expenses	$ 213.7	$ 129.2	$ 456.2	$ 247.8

Amount of earned premiums ceded to other entities.

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