Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Commercial - Workers Compensation (Details) - USD ($) $ in Millions	12 Months Ended
	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	Dec. 31, 2007
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 4,167	$ 4,089	$ 4,355
Liability for Unpaid Claims and Claims Adjustment Expense, Net	18,249	18,576	18,927	$ 19,117
Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	3,877
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	1,998
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	2,334
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	(30)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	4
Liability for Unpaid Claims and Claims Adjustment Expense, Net	4,306
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	535	533	522	502	$ 471	$ 429	$ 390	$ 337	$ 246	$ 100
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	520	505	489	461	425	381	323	233	$ 92
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	516	495	468	435	381	315	223	$ 88
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	565	531	500	433	352	245	$ 94
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	515	471	432	353	245	$ 97
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	465	411	338	229	$ 86
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	366	297	211	$ 79
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	213	157	$ 60
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	130	$ 50
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial - Workers Compensation [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 52