Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - 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, 2017	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, 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, Workers' Compensation	$ (65)	$ 150	$ 80
Workers' Compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(76)	106	71
Adjustment for development on a discounted basis	(4)	1	(2)
Total net development for accident years prior to 2008	14	43	11
Total unallocated claim adjustment expense development	1	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation	(65)	150	80
Workers' Compensation | 2008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	14	8	8	$ 10	$ 4	$ 2	$ 13	$ 18	$ 17								$ 94
Workers' Compensation | 2009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	7	8	6	11	4	2	7	$ 4								$ 49
Workers' Compensation | 2010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	4	20	14	20	22	22	$ 43								$ 145
Workers' Compensation | 2011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	24	(6)	11	9	$ 35								$ 75
Workers' Compensation | 2012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(4)	10	13	32	$ 27								$ 78
Workers' Compensation | 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(26)	26	21	$ 35								$ 56
Workers' Compensation | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(26)	0	$ 15								$ (11)
Workers' Compensation | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(25)	10								$ (15)
Workers' Compensation | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ (22)	$ (22)