Commitments, Contingencies and Other Items - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies [Line Items]
Initial commitments	$ 3,600.0	$ 2,900.0
Fulfilled commitments	2,000.0	1,700.0
Unfunded commitments	$ 1,600.0	$ 1,200.0
Operating lease, weighted average lease term	6 years 10 months 24 days
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets	Other assets
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Operating lease, right-of-use asset	$ 72.9	$ 66.6
Operating lease, liability	103.9	67.1
Operating lease, expense	$ 12.8	$ 8.8
Lessee, finance lease, term of contract	20 years
Lessee, finance lease, renewal term	30 years
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets	Other assets
Finance Lease, Liability, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Finance lease, right-of-use asset	$ 16.6	$ 17.1
Finance lease, liability	21.5	22.0
Letter of Credit
Commitments and Contingencies [Line Items]
Letters of credit outstanding	1,600.0
Letter of Credit | Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Line of credit facility, maximum borrowing capacity	37.5
Letter of Credit | Capital Support Agreement | Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Maximum mandatory capital contribution in the event of loss that reduced capital and surplus below specified level	50.0
Interest Expense
Commitments and Contingencies [Line Items]
Finance lease, interest expense	2.2	2.2
Other income (loss)
Commitments and Contingencies [Line Items]
Finance lease, right-of-use asset, amortization	$ 0.5	$ 0.5