Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions	Jan. 01, 2021	Oct. 01, 2020	Dec. 31, 2022	Dec. 31, 2021
Other Significant Transactions [Line Items]
Goodwill			$ 176.5	$ 142.3
Ark
Other Significant Transactions [Line Items]
Investments	$ 594.3
Cash	52.0
Reinsurance recoverables	433.4
Insurance premiums receivable	236.7
Ceded unearned premiums	170.2
Value of in-force business acquired	71.7
Other assets	88.9
Loss and loss adjustment expense reserves	(696.0)
Unearned insurance premiums	(326.1)
Debt	(46.4)
Ceded reinsurance payable	(528.3)
Other liabilities	(25.9)
Net tangible assets acquired	24.5
Goodwill	116.8
Other intangible assets - syndicate underwriting capacity	175.7
Deferred tax liability on other intangible assets	(33.4)
Net assets acquired	283.6
Merger agreement, equity capital contributed	$ 605.4	$ 605.4