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Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earned premiums:
Net earned premiums	$ 664.2	$ 22.8	$ 16.3
Ark
Written premiums:
Direct	1,058.7
Ceded	(199.6)
Net written premiums	859.1
Earned premiums:
Direct	886.4
Ceded	(249.1)
Net earned premiums	637.3
Losses and LAE:
Gross	442.9
Ceded	(128.1)
Net Losses and LAE	$ 314.8