Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Deferred Revenue	$ 97	$ 0
Accident and Health Insurance Product Line [Member]
Prior accident year development [1]	(72)	(90)
Change in Workers Compensation Discount Including Accretion [Member]
Prior accident year development [1]	27	25
General Liability [Member]
Prior accident year development [1]	112	62
Marine
Prior accident year development [1]	1	8
Package Business [Member]
Prior accident year development [1]	(24)	(32)
Property Insurance [Member] | Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	(6)	(16)
Property Insurance [Member] | Property and Casualty, Personal Insurance [Member]
Prior accident year development [1]	3	0
Professional Liability Insurance [Member]
Prior accident year development [1]	(16)	32
Surety Product Line [Member]
Prior accident year development [1]	(10)	(2)
Assumed reinsurance
Prior accident year development [1]	(7)	3
Automobiles [Member] | Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	27	27
Automobiles [Member] | Property and Casualty, Personal Insurance [Member]
Prior accident year development [1]	(53)	(28)
Catastrophes [Member]
Prior accident year development [1]	(413)	(27)
Uncollectible reinsurance
Prior accident year development [1]	(8)	0
Insurance, Other [Member]
Prior accident year development [1]	22	15
Property, Liability and Casualty Insurance Product Line
Prior accident year development [1]	(320)	(23)
Prior accident year development before change in deferred gain	$ (417)	$ (23)