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Reserve for Unpaid Losses and Loss Adjustment Expenses - Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reconciling Items
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	$ 312	$ 16	$ 0
Adverse Development Cover Navigators Group [Member]
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	102	16
Deferred Revenue	118
Catastrophes | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(529)	(42)	(49)
Net asbestos reserves
Prior accident years reserve development
Prior accident year development [1]	127
Net asbestos reserves | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(2)	0	0
Net environmental reserves
Prior accident years reserve development
Prior accident year development [1]	91
Net environmental reserves | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	0	0	0
Asbestos and Environmental
Prior accident years reserve development
Prior accident year development [1]	218
Change in Deferred Gain on Retroactive Reinsurance	210
Workers’ compensation | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(110)	(120)	(164)
Workers’ compensation discount accretion | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	35	33	40
General liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	237	61	52
Marine [Member] | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	3	8	0
Package business | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(58)	(47)	(26)
Professional liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(14)	29	(12)
Bond | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(19)	(3)	2
Assumed Reinsurance [Member] | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(6)	3	0
Uncollectible reinsurance | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(8)	(30)	22
Other | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	54	46	38
Property and Casualty Insurance Products
Prior accident years reserve development
Prior accident year development [1]	(136)	(65)	(167)
Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit	(448)	(81)	(167)
Property and Casualty Insurance Products | Adverse Development Cover Navigators Group [Member]
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	312	16	0
P&C Commercial Lines | Property | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	(4)	(11)	(12)
P&C Commercial Lines | Automobile liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	27	27	(15)
P&C Personal Lines | Property | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	7	3	(25)
P&C Personal Lines | Automobile liability | Segment Reconciling Items
Prior accident years reserve development
Prior accident year development [1]	$ (61)	$ (38)	$ (18)

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