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, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,854
IBNR	1,249
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	698	$ 688	$ 674	$ 676	$ 676	$ 651	$ 659	$ 647	$ 641	$ 607
IBNR	$ 42
Cumulative Number of Claims | claim	46,443
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 663	668	671	673	678	669	659	627	$ 601
IBNR	$ 62
Cumulative Number of Claims | claim	42,685
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 552	561	582	593	618	592	572	$ 537
IBNR	$ 92
Cumulative Number of Claims | claim	38,758
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 439	446	450	452	479	480	$ 467
IBNR	$ 97
Cumulative Number of Claims | claim	33,488
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 382	394	408	406	431	$ 422
IBNR	$ 116
Cumulative Number of Claims | claim	31,876
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 366	382	396	405	$ 426
IBNR	$ 121
Cumulative Number of Claims | claim	31,967
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 400	421	432	$ 440
IBNR	$ 97
Cumulative Number of Claims | claim	33,094
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 428	440	$ 450
IBNR	$ 129
Cumulative Number of Claims | claim	34,800
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 449	$ 452
IBNR	$ 181
Cumulative Number of Claims | claim	34,020
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 477
IBNR	$ 312
Cumulative Number of Claims | claim	24,980