Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Commercial - Workers Compensation (Details) - Commercial - Workers Compensation [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
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	$ 125	$ 69	$ 125
Short-duration Insurance Contracts, Discounted Liabilities, Aggregate Discount	1	(4)	1
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years	24	15	13
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation	150	80	139
Short-duration Insurance Contracts, Accident Year 2007 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	19	(2)	6	$ 1	$ 0	$ (1)	$ 8	$ 16	$ 12								$ 59
Short-duration Insurance Contracts, Accident Year 2008 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	8	8	10	4	2	13	18	$ 17								$ 80
Short-duration Insurance Contracts, Accident Year 2009 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	8	6	11	4	2	7	$ 4								$ 42
Short-duration Insurance Contracts, Accident Year 2010 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	20	14	20	22	22	$ 43								$ 141
Short-duration Insurance Contracts, Accident Year 2011 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	24	(6)	11	9	$ 35								$ 73
Short-duration Insurance Contracts, Accident Year 2012 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	10	13	32	$ 27								$ 82
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	26	21	$ 35								$ 82
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	0	$ 15								$ 15
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development	$ 10