Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Deferred Revenue	$ 83	$ 0
Accident and Health Insurance Product Line [Member]
Prior accident year development [1]	(38)	(50)
Change in Workers Compensation Discount Including Accretion [Member]
Prior accident year development [1]	18	17
General Liability [Member]
Prior accident year development [1]	114	43
Marine
Prior accident year development [1]	1	10
Package Business [Member]
Prior accident year development [1]	(6)	(9)
Property Insurance [Member] | Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	(2)	(15)
Property Insurance [Member] | Property and Casualty, Personal Insurance [Member]
Prior accident year development [1]	0	1
Professional Liability Insurance [Member]
Prior accident year development [1]	5	33
Surety Product Line [Member]
Prior accident year development [1]	(10)	0
Assumed reinsurance
Prior accident year development [1]	(7)	3
Automobiles [Member] | Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	27	2
Automobiles [Member] | Property and Casualty, Personal Insurance [Member]
Prior accident year development [1]	(21)	(5)
Catastrophes [Member]
Prior accident year development [1]	(413)	(22)
Uncollectible reinsurance
Prior accident year development [1]	(2)	0
Insurance, Other [Member]
Prior accident year development [1]	6	16
Property, Liability and Casualty Insurance Product Line
Prior accident year development [1]	(245)	24
Prior accident year development before change in deferred gain	$ (328)	$ 24