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Third Party Reinsurance - Schedule of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earned premiums:
Earned insurance premiums	$ 111.0	$ 5.4
Losses and Loss Adjustment Expense [Abstract]
Reinsurance recoverables on unpaid losses	447.5
Reinsurance recoverables on paid losses	$ 1.5
Percent of reinsurance recoverables from insurers with excellent or higher credit quality	90.00%
Ark
Written premiums:
Direct	$ 404.5
Ceded	(62.1)
Net written premiums	342.4
Earned premiums:
Direct	162.6
Ceded	(58.0)
Earned insurance premiums	104.6
Losses and Loss Adjustment Expense [Abstract]
Gross	115.6
Ceded	(49.6)
Net Losses and LAE	66.0
Amount attributable to TPC Providers
Losses and Loss Adjustment Expense [Abstract]
Reinsurance recoverables on unpaid losses	331.0
All Entities Except TCP Providers
Losses and Loss Adjustment Expense [Abstract]
Reinsurance recoverables on unpaid losses	$ 116.5

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