Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	$ (121)	$ (121)	$ (471)	$ (366)
Operating segments
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(121)	(121)	(471)	(366)
Operating segments | Short tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(156)	(163)	(511)	(439)
Operating segments | Long tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	35	42	40	73
Operating segments | Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(14)	(16)	(39)	(31)
Operating segments | Insurance | Short tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(27)	(31)	(55)	(61)
Operating segments | Insurance | Short tailed lines | Property, energy, marine and aviation
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(11)	(15)	(21)
Operating segments | Insurance | Short tailed lines | Travel and accident
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(8)	(11)	(23)	(29)
Operating segments | Insurance | Short tailed lines | Surety Product Line
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse				(31)
Operating segments | Insurance | Long tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	13	15	16	30
Operating segments | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(53)	(41)	(253)	(115)
Operating segments | Reinsurance | Short tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(75)	(68)	(277)	(158)
Operating segments | Reinsurance | Short tailed lines | Property other than property catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(28)	(20)	(117)	(71)
Operating segments | Reinsurance | Short tailed lines | Property catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(28)		(114)
Operating segments | Reinsurance | Short tailed lines | Specialty lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse		(35)		(72)
Operating segments | Reinsurance | Long tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	22	27	24	43
Operating segments | Reinsurance | Long tailed lines | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	22	27	24	43
Operating segments | Mortgage
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(54)	(64)	(179)	(220)
Operating segments | Mortgage | Short tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	(54)	(64)	(179)	(220)
Operating segments | Mortgage | Long tailed lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development (favorable) adverse	$ 0	$ 0	$ 0	$ 0