CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ 0.9	$ (210.4)	$ (148.1)	$ (175.2)
Foreign currency translation:
Foreign currency translation adjustments	0.1	1.0	(0.5)	(0.7)
Reclassification adjustment for foreign currency translation included in net income	0.0	0.0	0.0	31.8
Defined benefit pension plans:
Net gain (loss) arising during the period	0.8	1.0	0.4	(0.9)
Fixed maturity securities:
Unrealized gains (losses) arising during the year	64.7	15.2	(16.5)	(428.1)
Reclassification adjustment for (gains) losses included in net income	(0.1)	28.0	(8.7)	43.6
Other comprehensive (loss) income before tax	65.5	45.2	(25.3)	(354.3)
Defined benefit pension plans:
Net gain (loss) arising during the period	0.2	0.2	0.1	(0.2)
Fixed maturity securities:
Unrealized gains (losses) arising during the period	13.5	1.5	(3.5)	(80.9)
Reclassification adjustment for (gains) losses included in net income	0.0	5.9	(1.8)	9.2
Income tax provision (benefit) related to other comprehensive income (loss)	13.7	7.6	(5.2)	(71.9)
Other comprehensive income (loss), net of tax	51.8	37.6	(20.1)	(282.4)
Comprehensive income (loss)	$ 52.7	$ (172.8)	$ (168.2)	$ (457.6)