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, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	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 ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,232
IBNR	1,216
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	324	$ 334	$ 353	$ 372	$ 382	$ 394	$ 408	$ 406	$ 431	$ 422
IBNR	$ 45
Cumulative Number of Claims | claim	31,916
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 293	308	331	355	366	382	396	405	$ 426
IBNR	$ 45
Cumulative Number of Claims | claim	32,000
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 383	398	399	402	400	421	432	$ 440
IBNR	$ 65
Cumulative Number of Claims | claim	33,156
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 399	404	415	415	428	440	$ 450
IBNR	$ 65
Cumulative Number of Claims | claim	34,914
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 416	419	436	437	449	$ 452
IBNR	$ 67
Cumulative Number of Claims | claim	34,377
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 393	414	446	466	$ 477
IBNR	$ 107
Cumulative Number of Claims | claim	29,481
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 421	432	454	$ 468
IBNR	$ 116
Cumulative Number of Claims | claim	30,126
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 478	489	$ 497
IBNR	$ 148
Cumulative Number of Claims | claim	33,428
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 551	$ 555
IBNR	$ 233
Cumulative Number of Claims | claim	36,822
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 574
IBNR	$ 325
Cumulative Number of Claims | claim	34,332