Significant Transactions - Ark (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Jan. 01, 2021	Oct. 01, 2020	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2021
Other Significant Noncash Transactions [Line Items]
Change in fair value of contingent consideration				$ 1.0	$ (3.3)	$ 2.1
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				39.1	36.4
Ark
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction					366.3
Goodwill and other intangible assets				292.5	0.0
Contingent consideration				28.0	0.0
Change in fair value of contingent consideration				5.5
Ark | In-force Business Acquired
Other Significant Noncash Transactions [Line Items]
Amortization				$ 64.2
Ark | Lloyd
Other Significant Noncash Transactions [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction					$ 280.0
Ark
Other Significant Noncash Transactions [Line Items]
Merger agreement, equity capital contributed	$ 605.4	$ 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	$ 200.0
Ownership interest after all transactions (as a percent)	72.00%
Ownership interest on fully-diluted basis (as a percent)	63.00%
Percent of shares outstanding	12.50%			12.50%
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
Ark | Group Ark Insurance Limited | Unsecured Subordinated Debt
Other Significant Noncash Transactions [Line Items]
Notes issued							$ 163.3