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, 2017 USD ($) claims	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 5,606
IBNR	958
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	652	$ 638	$ 630	$ 622	$ 612	$ 608	$ 606	$ 593	$ 575	$ 558
IBNR	$ 34
Cumulative Number of Claims | claims	59,911
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 632	625	617	611	600	596	594	587	$ 583
IBNR	$ 34
Cumulative Number of Claims | claims	51,161
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 721	717	697	683	663	641	619	$ 576
IBNR	$ 33
Cumulative Number of Claims | claims	48,144
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 668	666	642	648	637	628	$ 593
IBNR	$ 23
Cumulative Number of Claims | claims	44,691
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 667	671	661	648	616	$ 589
IBNR	$ 63
Cumulative Number of Claims | claims	41,756
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 584	610	584	563	$ 528
IBNR	$ 57
Cumulative Number of Claims | claims	38,153
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 448	474	474	$ 459
IBNR	$ 106
Cumulative Number of Claims | claims	33,072
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 401	426	$ 416
IBNR	$ 154
Cumulative Number of Claims | claims	31,470
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 399	$ 421
IBNR	$ 200
Cumulative Number of Claims | claims	31,310
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 434
IBNR	$ 254
Cumulative Number of Claims | claims	27,929