Reserve for Unpaid Losses and Loss Adjustment Expenses - Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reconciling Items
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	$ 246	$ 312	$ 16
Adverse Development Cover Navigators Group [Member]
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	91	102	16
Catastrophes | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(154)	(529)	(42)
Asbestos and Environmental
Prior accident years reserve development
Prior accident year development [1]		218
Change in Deferred Gain on Retroactive Reinsurance	155	210
Asbestos and Environmental | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	0	(2)	0
Workers’ compensation | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(190)	(110)	(120)
Workers’ compensation discount accretion | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	35	35	33
General liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	454	237	61
Marine [Member] | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	1	3	8
Package business | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(91)	(58)	(47)
Professional liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(2)	(14)	29
Bond | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(26)	(19)	(3)
Assumed Reinsurance [Member] | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(6)	(6)	3
Uncollectible reinsurance | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(6)	(8)	(30)
Other | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	42	54	46
Property and Casualty Insurance Products
Prior accident years reserve development
Prior accident year development [1]	199	(136)	(65)
Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit	(47)	(448)	(81)
Property and Casualty Insurance Products | Adverse Development Cover Navigators Group [Member]
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	246	312	16
P&C Commercial Lines | Property | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(26)	(4)	(11)
P&C Commercial Lines | Automobile liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	9	27	27
P&C Personal Lines | Property | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	3	7	3
P&C Personal Lines | Automobile liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	$ (90)	$ (61)	$ (38)