Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Commercial - Auto (Details) - Commercial - Auto [Member] $ in Millions	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 ($)	Dec. 31, 2007 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 247
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 2,630
Short-duration Insurance Contracts, Accident Year 2007 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	67,473
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 352	$ 354	$ 355	$ 356	$ 358	$ 355	$ 360	$ 368	$ 367	$ 348
Short-duration Insurance Contracts, Accident Year 2008 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	56,407
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 296	298	298	305	309	306	316	323	$ 322
Short-duration Insurance Contracts, Accident Year 2009 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	47,325
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 272	275	277	281	278	274	272	$ 287
Short-duration Insurance Contracts, Accident Year 2010 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	46,324
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 281	286	291	283	279	274	$ 262
Short-duration Insurance Contracts, Accident Year 2011 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	46,676
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 285	290	293	279	273	$ 262
Short-duration Insurance Contracts, Accident Year 2012 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	45,279
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 11
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 300	296	292	282	$ 270
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	38,513
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 20
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 241	257	259	$ 242
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	32,958
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 40
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 210	221	$ 231
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	29,714
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 65
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 197	$ 199
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Number of Reported Claims per Million of Net Earned Premium	25,196
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 105
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 196