Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 839
IBNR	418
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	41	$ 42	$ 43	$ 45	$ 45	$ 60	$ 69	$ 94	$ 124	$ 123
IBNR	$ 2
Cumulative Number of Claims | claim	5,135
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 45	45	53	58	63	79	104	131	$ 131
IBNR	$ 2
Cumulative Number of Claims | claim	5,085
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 43	58	64	67	84	109	124	$ 124
IBNR	$ 3
Cumulative Number of Claims | claim	5,565
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 67	66	71	84	103	115	$ 120
IBNR	$ 4
Cumulative Number of Claims | claim	5,883
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 51	56	62	91	108	$ 114
IBNR	$ 10
Cumulative Number of Claims | claim	6,249
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 82	87	98	112	$ 119
IBNR	$ 12
Cumulative Number of Claims | claim	6,152
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 67	81	119	$ 128
IBNR	$ 34
Cumulative Number of Claims | claim	4,678
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 110	129	$ 137
IBNR	$ 68
Cumulative Number of Claims | claim	4,645
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 158	$ 155
IBNR	$ 116
Cumulative Number of Claims | claim	4,350
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 175
IBNR	$ 167
Cumulative Number of Claims | claim	2,750