Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - 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, General Liability	$ 0	$ (28)	$ (33)
General Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(5)	(13)	(49)
Total net development for accident years prior to 2008	(2)	(15)	16
Total unallocated claim adjustment expense development	7	0	0
Net prior year claim and allocated claim adjustment expense reserve development, General Liability	0	(28)	(33)
General Liability | 2008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	(9)	(13)	$ (19)	$ (1)	$ (14)	$ 17	$ 26	$ (7)								$ (19)
General Liability | 2009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	5	3	(4)	(6)	(4)	(1)	(3)	$ 46								$ 36
General Liability | 2010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(5)	(16)	(20)	46	50	2	$ 31								$ 88
General Liability | 2011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(6)	10	1	46	30	$ (3)								$ 78
General Liability | 2012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	11	(12)	(9)	16	$ 24								$ 30
General Liability | 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(39)	9	(11)	$ 30								$ (11)
General Liability | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(8)	1	$ 7								$ 0
General Liability | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	11	1								$ 12
General Liability | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 25	$ 25