Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Commercial - General Liability (Details) - Commercial - General Liability [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	$ 1,382
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 6,065
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	53,553
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 28
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 540	$ 548	$ 545	$ 559	$ 600	$ 610	$ 639	$ 678	$ 673	$ 774
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	44,586
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 18
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 591	600	613	632	633	647	630	604	$ 611
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	43,955
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 16
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 622	619	623	629	633	634	637	$ 591
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	43,378
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 659	675	695	649	599	597	$ 566
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	38,101
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 621	611	610	564	534	$ 537
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	34,037
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 63
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 558	570	579	563	$ 539
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	32,897
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 142
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 643	634	645	$ 615
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	26,744
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 224
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 635	634	$ 627
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	21,687
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 330
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 574	$ 573
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	16,720
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 495
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 622