Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Loss and LAE incurred relating to:
Net incurred losses and LAE	$ 265.8	$ 213.7	$ 581.1	$ 456.2
Syndicate - Ark Insurance Holdings Limited
Loss and LAE paid relating to:
Percent of underwriting capital provided by third parties					0.00%	42.80%	58.30%
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,421.0	1,022.1	1,296.5	894.7
Less: beginning reinsurance recoverable on unpaid losses	(362.7)	(375.5)	(505.0)	(428.9)
Net loss and LAE reserves	1,058.3	646.6	791.5	465.8
Loss and LAE incurred relating to:
Current year losses	266.4	222.9	562.1	491.8
Prior year losses	(0.6)	(9.2)	19.0	(35.6)
Net incurred losses and LAE	265.8	213.7	581.1	456.2
Loss and LAE paid relating to:
Current year losses	(8.8)	(6.1)	(18.4)	(20.8)
Prior year losses	(90.4)	(37.0)	(281.1)	(130.8)
Net paid loss and LAE	(99.2)	(43.1)	(299.5)	(151.6)
Change in TPC providers’ participation	0.0	0.0	145.4	57.5
Foreign currency translation and other adjustments to loss and LAE reserves	(5.5)	(9.2)	0.9	(19.9)
Net ending balance	1,219.4	808.0	1,219.4	808.0
Plus: ending reinsurance recoverable on unpaid losses	352.4	521.4	352.4	521.4
Gross ending balance	1,571.8	1,329.4	1,571.8	1,329.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	(0.0)	(182.4)	(145.4)	(276.8)
Loss and LAE paid relating to:
Reinsurance recoverables	$ 0.0	$ 168.0	$ 0.0	$ 168.0