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, 2018 USD ($) claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 5,445
IBNR	1,130
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	650	$ 649	$ 638	$ 631	$ 626	$ 616	$ 611	$ 609	$ 599	$ 592
IBNR	$ 46
Cumulative Number of Claims | claims	51,822
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 732	733	730	710	698	676	654	632	$ 583
IBNR	$ 44
Cumulative Number of Claims | claims	49,106
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 674	676	676	651	659	647	641	$ 607
IBNR	$ 27
Cumulative Number of Claims | claims	45,637
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 671	673	678	669	659	627	$ 601
IBNR	$ 59
Cumulative Number of Claims | claims	42,477
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 582	593	618	592	572	$ 537
IBNR	$ 86
Cumulative Number of Claims | claims	38,665
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 450	452	479	480	$ 467
IBNR	$ 104
Cumulative Number of Claims | claims	33,465
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 408	406	431	$ 422
IBNR	$ 146
Cumulative Number of Claims | claims	31,828
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 396	405	$ 426
IBNR	$ 171
Cumulative Number of Claims | claims	31,905
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 432	$ 440
IBNR	$ 171
Cumulative Number of Claims | claims	32,811
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 450
IBNR	$ 276
Cumulative Number of Claims | claims	30,399