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Significant Transactions - Ark (Details) - USD ($) $ in Millions			3 Months Ended
	Jan. 01, 2021	Oct. 01, 2020	Mar. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Ark
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction					$ 366.3
Ark
Other Significant Noncash Transactions [Line Items]
Ownership interest (as a percent)				72.00%
Goodwill and other intangible assets				$ 292.5	0.0
Contingent consideration				$ 24.0	0.0
Ark | Ark
Other Significant Noncash Transactions [Line Items]
Fully diluted ownership interest (as a percent)				63.00%
Ark | Lloyd
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction					280.0
Other Operations
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction				$ 0.0	646.3
Goodwill and other intangible assets				48.7	$ 36.4
Ark
Other Significant Noncash Transactions [Line Items]
Merger agreement, equity capital contributed	$ 605.4
Equity capital, pre money valuation amount		$ 300.0
Additional equity purchase from shareholders		$ 40.9
Investment, potential additional contribution of equity capital	200.0
Assets acquired	2,500.0
Goodwill and other intangible assets	292.5
Liabilities acquired	1,700.0
Contingent consideration	22.5
Noncontrolling interest acquired	$ 220.2
Transaction costs			$ 25.3
Investment, potential additional contribution of equity capital terminated				$ 200.0
Ark | Ark
Other Significant Noncash Transactions [Line Items]
Ownership interest (as a percent)				72.00%
Fully diluted ownership interest (as a percent)				63.00%
Percent of outstanding shares after performance obligations are satisfied				12.50%
Ark | Group Ark Insurance Limited | Unsecured Subordinated Debt
Other Significant Noncash Transactions [Line Items]
Notes issued				$ 163.3

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