Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Workers’ compensation
Product Information [Line Items]
Prior accident year development	$ (20)	$ (25)
Workers’ compensation discount accretion
Product Information [Line Items]
Prior accident year development	8	10
General liability
Product Information [Line Items]
Prior accident year development	6	8
Package business
Product Information [Line Items]
Prior accident year development	5	8
Professional liability
Product Information [Line Items]
Prior accident year development	0	2
Bond
Product Information [Line Items]
Prior accident year development	0	0
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	(8)	(3)
Uncollectible reinsurance
Product Information [Line Items]
Prior accident year development	0	0
Other reserve re-estimates, net
Product Information [Line Items]
Prior accident year development	4	(2)
Property and Casualty, Commercial Insurance Product Line [Member] | Property
Product Information [Line Items]
Prior accident year development	(2)	(13)
Property and Casualty, Commercial Insurance Product Line [Member] | Automobile
Product Information [Line Items]
Prior accident year development	0	(5)
Property and Casualty, Personal Insurance Product Line [Member] | Property
Product Information [Line Items]
Prior accident year development	1	(12)
Property and Casualty, Personal Insurance Product Line [Member] | Automobile
Product Information [Line Items]
Prior accident year development	$ (5)	$ 0