Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Specialty - Other Professional and Management Liability (Details) - Specialty - Other Professional and Management Liability [Member] - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 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, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	$ (134)	$ 7	$ (52)
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years	4	(7)	(35)
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability	(130)	0	(87)
Short-duration Insurance Contracts, Accident Year 2007 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	0	(19)	(23)	$ (20)	$ (10)	$ (20)	$ (52)	$ (11)	$ 13								$ (142)
Short-duration Insurance Contracts, Accident Year 2008 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(18)	(5)	(30)	(35)	(9)	(30)	21	$ 17								$ (89)
Short-duration Insurance Contracts, Accident Year 2009 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	8	(5)	9	(7)	(5)	30	$ 44								$ 74
Short-duration Insurance Contracts, Accident Year 2010 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(13)	(10)	(2)	(2)	23	$ 2								$ (2)
Short-duration Insurance Contracts, Accident Year 2011 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(34)	(4)	15	29	$ 28								$ 34
Short-duration Insurance Contracts, Accident Year 2012 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(37)	(6)	(18)	$ (13)								$ (74)
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(39)	38	$ (3)								$ (4)
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	(5)	$ 18								$ 13
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Claims Development [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	$ 4