Business and Significant Accounting Policies - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 5.3	$ 28.3	$ 35.6	$ 24.1
Income taxes paid	0.1	0.2	0.1	26.2
Income taxes recovered	0.0	(0.1)	(32.0)	(2.1)
Income taxes paid, net	0.1	0.1	(31.9)	24.1
Senior unsecured fixed rate notes
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	1.2	8.2	9.3	9.3
Junior subordinated debentures
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	4.1	19.3	24.4	13.5
Other indebtedness
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 0.0	$ 0.8	$ 1.9	$ 1.3