SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS (Details) - Reinsurance recoverables: Allowance for uncollectible balances - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
VALUATION AND QUALIFYING ACCOUNTS
Balance at beginning of period	$ 7.8	$ 3.6	$ 2.3
Charged to costs and expenses	(0.2)	4.6	1.3
Charged to other accounts	0.0	0.0	0.0
Deductions described	0.1	(0.4)	0.0
Balance at end of period	$ 7.7	$ 7.8	$ 3.6