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Third Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earned premiums:
Gross earned premiums (1)	$ 220.1	$ 6.2	$ 455.4	$ 17.2
Ark
Written premiums:
Gross	162.4		895.0
Ceded	(41.5)		(169.5)
Gross written premiums (1)	120.9		725.5
Earned premiums:
Gross	284.5		622.0
Ceded	(71.1)		(186.2)
Gross earned premiums (1)	213.4		435.8
Losses and Loss Adjustment Expense [Abstract]
Gross	193.5		372.7
Ceded	(64.3)		(124.9)
Net Losses and LAE	$ 129.2		$ 247.8

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