Segment Information - Reconciliation of Revenue to Income (Loss) Before Income Taxes (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2024	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Net earned premiums		$ 162.3	$ 1,225.9	$ 1,089.8	$ 1,740.4
Net investment income		28.6	121.3	249.8	129.8
Net investment and other gains (losses)		(0.3)	(22.7)	20.0	(115.3)
Total revenue		190.6	1,324.5	1,359.6	1,754.9
Losses and loss adjustment expenses	$ 94.2	94.2	1,043.2	1,018.3	1,166.9
Underwriting, acquisition and general expenses		77.2	419.0	477.0	670.7
Non-operating expense		13.1	41.1	17.6	51.5
Foreign currency exchange (gains) losses		0.6	1.8	(1.0)	(5.0)
Net investment and other (gains) losses		0.3	22.7	(20.0)	115.3
Total other expenses		39.3		174.3
Income (loss) before income taxes		2.2	(210.1)	(190.3)	(183.2)
Amortization of intangible assets		37.9	0.0	172.0	0.0
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Net earned premiums		162.3		1,089.8
Net investment income		28.6		249.8
Total revenue		190.9		1,339.6
Segment operating income (loss)		55.5		(19.4)
Income (loss) before income taxes			(122.0)		39.1
Casualty Lines | Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Net earned premiums		66.5		546.1
Net investment income		10.1		112.4
Total revenue		76.6		658.5
Losses and loss adjustment expenses		42.1		432.2
Underwriting, acquisition and general expenses		15.2		145.5
Other segment items		1.3		17.2
Segment operating income (loss)		18.0		63.6
Specialty Lines | Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Net earned premiums		33.2		257.8
Net investment income		5.2		17.5
Total revenue		38.4		275.3
Losses and loss adjustment expenses		19.5		165.7
Underwriting, acquisition and general expenses		9.5		66.2
Other segment items		0.6		2.7
Segment operating income (loss)		8.8		40.7
Run-off Lines | Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Net earned premiums		62.6	0.3	285.9	0.9
Net investment income		13.3	$ 2.7	119.9	$ 2.3
Total revenue		75.9		405.8
Losses and loss adjustment expenses		32.6		420.4
Underwriting, acquisition and general expenses		13.2		91.0
Other segment items		1.4		18.1
Segment operating income (loss)		$ 28.7		$ (123.7)