Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,163	$ 16,929	$ 16,300	$ 15,857
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	9,021	8,430
Workers' Compensation | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	2,418
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,754
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2014	1,842
Other	(23)
Liability for unallocated claim adjustment expenses for accident years presented	72
Total net liability for unpaid claim and claim adjustment expenses	3,645
Workers' Compensation | 2014 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	319	312	306	297	$ 290	$ 282	$ 258	$ 215	$ 159	$ 61
Workers' Compensation | 2015 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	259	256	251	243	231	212	180	131	$ 51
Workers' Compensation | 2016 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	235	234	227	219	198	169	129	$ 53
Workers' Compensation | 2017 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	287	279	265	243	207	151	$ 63
Workers' Compensation | 2018 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	298	280	259	229	163	$ 68
Workers' Compensation | 2019 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	291	262	223	169	$ 71
Workers' Compensation | 2020 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	228	200	147	$ 65
Workers' Compensation | 2021 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	222	164	$ 67
Workers' Compensation | 2022 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	192	$ 79
Workers' Compensation | 2023 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 87