Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	1 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,244.8	$ 1,232.9	$ 1,297.5
Other comprehensive income (loss) before reclassifications	51.9	15.5	(13.2)
Amounts reclassified from accumulated other comprehensive income	(0.1)	22.1	(6.9)
Net current-period other comprehensive income (loss)	51.8	37.6	(20.1)
Ending Balance	1,297.5	1,049.9	1,603.5
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0	(305.1)	51.8
Ending Balance	51.8	(267.5)	31.7
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0	(4.2)	0.1
Other comprehensive income (loss) before reclassifications	0.1	1.0	(0.5)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0	0.0
Net current-period other comprehensive income (loss)	0.1	1.0	(0.5)
Ending Balance	0.1	(3.2)	(0.4)
Unrealized Holding Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0	(293.1)	51.1
Other comprehensive income (loss) before reclassifications	51.2	13.7	(13.0)
Amounts reclassified from accumulated other comprehensive income	(0.1)	22.1	(6.9)
Net current-period other comprehensive income (loss)	51.1	35.8	(19.9)
Ending Balance	51.1	(257.3)	31.2
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0	(7.8)	0.6
Other comprehensive income (loss) before reclassifications	0.6	0.8	0.3
Amounts reclassified from accumulated other comprehensive income	0.0	0.0	0.0
Net current-period other comprehensive income (loss)	0.6	0.8	0.3
Ending Balance	$ 0.6	$ (7.0)	$ 0.9