Accumulated Other Comprehensive Income (Loss) - Schedule of Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Unrealized gains and losses on securities:
Net realized investment and other gains (losses)	$ (0.2)	$ (4.7)	$ 30.8	$ (21.0)
Income tax provision (benefit)	(34.4)	17.6	(27.2)	3.3
Net income (loss)	(132.3)	(46.9)	(94.0)	(78.5)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net income (loss)	(1.3)	(1.9)	3.8	(22.4)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and Losses on Securities
Unrealized gains and losses on securities:
Net realized investment and other gains (losses)	(1.6)	(1.5)	4.8	(27.4)
Income tax provision (benefit)	$ 0.3	$ (0.4)	$ (1.0)	$ 5.0