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Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions	Mar. 31, 2021	Jan. 01, 2021	Dec. 31, 2020
Other Significant Transactions [Line Items]
Goodwill	$ 613.4		$ 525.5
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

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