Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Cumulative Paid Claim and Claim Adjustment Expensess) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
Workers' Compensation | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,780
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,826
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	2,216
Other	(37)
Liability for unallocated claim adjustment expenses for accident years presented	7
Total net liability for unpaid claim and claim adjustment expenses	4,012
Workers' Compensation | 2008 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	541	520	505	489	$ 461	$ 425	$ 381	$ 323	$ 233	$ 92
Workers' Compensation | 2009 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	539	516	495	468	435	381	315	223	$ 88
Workers' Compensation | 2010 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	603	565	531	500	433	352	245	$ 94
Workers' Compensation | 2011 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	557	515	471	432	353	245	$ 97
Workers' Compensation | 2012 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	503	465	411	338	229	$ 86
Workers' Compensation | 2013 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	413	366	297	211	$ 79
Workers' Compensation | 2014 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	256	213	157	$ 60
Workers' Compensation | 2015 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	179	130	$ 50
Workers' Compensation | 2016 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	127	$ 52
Workers' Compensation | 2017 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 62