Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	$ 894.7	$ 696.0
Less: beginning reinsurance recoverable on unpaid losses	(428.9)	(433.4)
Net loss and loss adjustment expense reserves	465.8	262.6
Loss and loss adjustment expenses incurred relating to:
Current year losses	125.7	65.5
Prior year losses	(3.7)	0.5
Net incurred losses and loss adjustment expenses	122.0	66.0
Loss and loss adjustment expenses paid relating to:
Current year losses	(0.6)	(5.2)
Prior year losses	(39.4)	(19.7)
Net loss and loss adjustment expenses	(40.0)	(24.9)
Change in TPC providers’ participation	57.5	(2.2)
Foreign currency translation and other adjustments to loss and loss adjustment expense reserves	(1.7)	2.9
Net ending balance	603.6	304.4
Plus: ending reinsurance recoverable on unpaid losses	396.0	447.5
Gross ending balance	999.6	751.9
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	396.0	447.5	$ 433.4
TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	(276.8)	(319.2)
Loss and loss adjustment expenses paid relating to:
Plus: ending reinsurance recoverable on unpaid losses	207.3	331.0
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 207.3	$ 331.0	$ 319.2
Ark | TPC Providers
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of underwriting capital provided by third parties			42.80%	58.30%	57.60%