Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - Commercial $ in Millions	Dec. 31, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,575
IBNR	1,325
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	664	$ 663	$ 668	$ 671	$ 673	$ 678	$ 669	$ 659	$ 627	$ 601
IBNR	$ 66
Cumulative Number of Claims | claim	42,804
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 548	552	561	582	593	618	592	572	$ 537
IBNR	$ 91
Cumulative Number of Claims | claim	38,867
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 448	439	446	450	452	479	480	$ 467
IBNR	$ 105
Cumulative Number of Claims | claim	33,502
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 372	382	394	408	406	431	$ 422
IBNR	$ 101
Cumulative Number of Claims | claim	31,894
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 355	366	382	396	405	$ 426
IBNR	$ 104
Cumulative Number of Claims | claim	31,981
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 402	400	421	432	$ 440
IBNR	$ 92
Cumulative Number of Claims | claim	33,121
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 415	428	440	$ 450
IBNR	$ 104
Cumulative Number of Claims | claim	34,851
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 437	449	$ 452
IBNR	$ 134
Cumulative Number of Claims | claim	34,248
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 466	$ 477
IBNR	$ 228
Cumulative Number of Claims | claim	29,188
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 468
IBNR	$ 300
Cumulative Number of Claims | claim	25,711