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, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 5,187
IBNR	1,208
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	735	$ 732	$ 733	$ 730	$ 710	$ 698	$ 676	$ 654	$ 632	$ 583
IBNR	$ 55
Cumulative Number of Claims | claim	49,333
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 688	674	676	676	651	659	647	641	$ 607
IBNR	$ 40
Cumulative Number of Claims | claim	45,959
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 668	671	673	678	669	659	627	$ 601
IBNR	$ 67
Cumulative Number of Claims | claim	42,586
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 561	582	593	618	592	572	$ 537
IBNR	$ 93
Cumulative Number of Claims | claim	38,688
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 446	450	452	479	480	$ 467
IBNR	$ 99
Cumulative Number of Claims | claim	33,480
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 394	408	406	431	$ 422
IBNR	$ 130
Cumulative Number of Claims | claim	31,861
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 382	396	405	$ 426
IBNR	$ 144
Cumulative Number of Claims | claim	31,945
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 421	432	$ 440
IBNR	$ 138
Cumulative Number of Claims | claim	33,029
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 440	$ 450
IBNR	$ 185
Cumulative Number of Claims | claim	34,647
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 452
IBNR	$ 257
Cumulative Number of Claims | claim	29,795