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Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions	Oct. 01, 2020	Mar. 31, 2022	Dec. 31, 2021	Jan. 01, 2021
Other Significant Transactions [Line Items]
Goodwill		$ 641.4	$ 645.5
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

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