Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - 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	120 Months Ended
	Dec. 31, 2018	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, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto	$ 1	$ (35)	$ (47)
Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(41)	(37)
Total net development for accident years prior to 2009	0	4	(10)
Total unallocated claim adjustment expense development	1	2	0
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto	1	(35)	(47)
2009 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	0	(2)	$ (3)	$ (4)	$ 3	$ 2	$ 4	$ (15)									$ (14)
2010 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(1)	(4)	(5)	7	4	4	$ 16									$ 21
2011 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	0	(6)	(2)	14	7	$ 13									$ 26
2012 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(8)	4	4	10	$ 14									$ 24
2013 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(4)	(16)	0	$ 19									$ (1)
2014 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(7)	(11)	$ (11)									$ (29)
2015 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(9)	$ (2)									$ (11)
2016 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	$ (12)									$ (12)
2017 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ (1)									$ (1)