Schedule V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Valuation allowance for deferred tax assets
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 322.4	$ 283.0
Additions
Charged to costs and expenses	41.6	39.4
Charge to other accounts	0.0	0.0
(Deductions)	0.0	0.0
Balance at end of period	364.0	322.4
Allowance for premium receivables
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	2.5	2.7
Additions
Charged to costs and expenses	0.3	0.0
Charge to other accounts	0.0	0.0
(Deductions)	0.0	(0.2)
Balance at end of period	$ 2.8	$ 2.5