Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - Commercial $ 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	$ 4,172
IBNR	1,238
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	419	$ 430	$ 448	$ 439	$ 446	$ 450	$ 452	$ 479	$ 480	$ 467
IBNR	$ 70
Cumulative Number of Claims | claim	33,550
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 334	353	372	382	394	408	406	431	$ 422
IBNR	$ 59
Cumulative Number of Claims | claim	31,904
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 308	331	355	366	382	396	405	$ 426
IBNR	$ 56
Cumulative Number of Claims | claim	31,994
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 398	399	402	400	421	432	$ 440
IBNR	$ 78
Cumulative Number of Claims | claim	33,142
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 404	415	415	428	440	$ 450
IBNR	$ 74
Cumulative Number of Claims | claim	34,886
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 419	436	437	449	$ 452
IBNR	$ 78
Cumulative Number of Claims | claim	34,349
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 414	446	466	$ 477
IBNR	$ 135
Cumulative Number of Claims | claim	29,454
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 432	454	$ 468
IBNR	$ 146
Cumulative Number of Claims | claim	30,066
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 489	$ 497
IBNR	$ 198
Cumulative Number of Claims | claim	33,229
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 555
IBNR	$ 344
Cumulative Number of Claims | claim	31,549