Reserve for Unpaid Losses and Loss Adjustment Expenses - Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Workers’ compensation
Prior accident years reserve development
Prior accident year development	$ (79)	$ (119)	$ (37)
Workers’ compensation discount accretion
Prior accident years reserve development
Prior accident year development	28	28	29
Liability
Prior accident years reserve development
Prior accident year development	11	65	8
Package business
Prior accident years reserve development
Prior accident year development	(25)	65	28
Commercial property
Prior accident years reserve development
Prior accident year development	(8)	1	(6)
Professional liability
Prior accident years reserve development
Prior accident year development	1	(37)	(36)
Bond
Prior accident years reserve development
Prior accident year development	32	(8)	(2)
Commercial automobile liability
Prior accident years reserve development
Prior accident year development	17	57	62
Automobile liability
Prior accident years reserve development
Prior accident year development	0	160	(8)
Homeowners
Prior accident years reserve development
Prior accident year development	(14)	(10)	9
Net asbestos reserves
Prior accident years reserve development
Prior accident year development	0	197	146
Net environmental reserves
Prior accident years reserve development
Prior accident year development	0	71	55
Catastrophes
Prior accident years reserve development
Prior accident year development	(16)	(7)	(18)
Uncollectible reinsurance
Prior accident years reserve development
Prior accident year development	(15)	(30)	0
Other reserve re-estimates, net
Prior accident years reserve development
Prior accident year development	27	24	20
Property and Casualty Insurance Products
Prior accident years reserve development
Prior accident year development	$ (41)	$ 457	$ 250