Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Workers’ compensation
Product Information [Line Items]
Prior accident year development	$ (97)	$ (29)
Workers’ compensation discount accretion
Product Information [Line Items]
Prior accident year development	30	21
General liability
Product Information [Line Items]
Prior accident year development	32	10
Package business
Product Information [Line Items]
Prior accident year development	(16)	(22)
Professional liability
Product Information [Line Items]
Prior accident year development	(12)	0
Bond
Product Information [Line Items]
Prior accident year development	0	10
Net asbestos reserves
Product Information [Line Items]
Prior accident year development	0	0
Net environmental reserves
Product Information [Line Items]
Prior accident year development	0	0
Catastrophes
Product Information [Line Items]
Prior accident year development	(47)	(12)
Uncollectible reinsurance
Product Information [Line Items]
Prior accident year development	22	0
Other reserve re-estimates, net
Product Information [Line Items]
Prior accident year development	4	8
Property and Casualty, Commercial Insurance Product Line [Member] | Property
Product Information [Line Items]
Prior accident year development	(10)	(5)
Property and Casualty, Commercial Insurance Product Line [Member] | Automobile
Product Information [Line Items]
Prior accident year development	(15)	20
Property and Casualty, Personal Insurance Product Line [Member] | Property
Product Information [Line Items]
Prior accident year development	(20)	0
Property and Casualty, Personal Insurance Product Line [Member] | Automobile
Product Information [Line Items]
Prior accident year development	$ (10)	$ 0