Schedule V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Valuation allowance for deferred tax assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$ 283.0	$ 139.0
Additions
Charged to costs and expenses	39.4	144.0
Charge to other accounts	0.0	0.0
(Deductions)	0.0	0.0
Balance at end of period	322.4	283.0
Allowance for premium receivables
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	2.7	1.6
Additions
Charged to costs and expenses	0.0	1.1
Charge to other accounts	0.0	0.0
(Deductions)	(0.2)	0.0
Balance at end of period	$ 2.5	$ 2.7