Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Incurred Claims and Allocated Claim Adjustment Expenses Specialty - Medical Professional Liability (Details) - Specialty - Medical Professional 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	$ 1,054
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 4,872
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	12,122
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 385	$ 399	$ 401	$ 390	$ 391	$ 395	$ 427	$ 444	$ 452	$ 448
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	14,094
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 442	455	467	468	468	480	496	451	$ 426
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	15,573
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 12
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 432	463	471	480	506	494	469	$ 462
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	15,206
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	$ 403	446	470	486	478	478	$ 483
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	17,428
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 23
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 491	501	533	507	492	$ 486
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	18,375
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 559	567	575	529	$ 526
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	19,565
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 95
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 567	560	540	$ 534
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	19,286
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 165
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 585	548	$ 511
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	16,798
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 278
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 539	$ 480
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	11,600
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 400
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	$ 469