Losses and Loss Adjustment Expenses - Prior Period Development (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	$ (8)	$ (6)
Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	(8)	(6)
Run-off \| Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	0	(24)
Run-off \| Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	0	25
Run-off \| General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	17	18
Run-off \| Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	(5)	(2)
Run-off \| Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	1	2
Run-off \| Professional indemnity/ Directors and officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	(5)	(29)
Run-off \| Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	(7)	4
Run-off \| Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	0	(1)
Run-off \| All other
Liability for Claims and Claims Adjustment Expense [Line Items]
Reduction in estimates of net ultimate losses	$ (9)	$ 1