Reserve for Unpaid Losses and Loss Adjustment Expenses - Prior Accident Years Reserve Development (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Prior accident years reserve development
Change in Deferred Gain on Retroactive Reinsurance	$ 16,000,000	$ 0	$ 0
Workers’ compensation
Prior accident years reserve development
Prior accident year development [1]	(120,000,000)	(164,000,000)	(79,000,000)
Workers’ compensation discount accretion
Prior accident years reserve development
Prior accident year development [1]	33,000,000	40,000,000	28,000,000
General liability
Prior accident years reserve development
Prior accident year development [1]	61,000,000	52,000,000	11,000,000
Marine [Member]
Prior accident years reserve development
Prior accident year development [1]	8,000,000	0	0
Package business
Prior accident years reserve development
Prior accident year development [1]	(47,000,000)	(26,000,000)	(25,000,000)
Property
Prior accident years reserve development
Prior accident year development [1]	3,000,000	(25,000,000)	(14,000,000)
Professional liability
Prior accident years reserve development
Prior accident year development [1]	29,000,000	(12,000,000)	1,000,000
Bond
Prior accident years reserve development
Prior accident year development [1]	(3,000,000)	2,000,000	32,000,000
Assumed Reinsurance [Member]
Prior accident years reserve development
Prior accident year development [1]	3,000,000	0	0
Net asbestos reserves
Prior accident years reserve development
Prior accident year development [1]	0	0	0
Net environmental reserves
Prior accident years reserve development
Prior accident year development [1]	0	0	0
Catastrophes
Prior accident years reserve development
Prior accident year development [1]	(42,000,000)	(49,000,000)	(16,000,000)
Uncollectible reinsurance
Prior accident years reserve development
Prior accident year development [1]	(30,000,000)	22,000,000	(15,000,000)
Other
Prior accident years reserve development
Prior accident year development [1]	46,000,000	38,000,000	27,000,000
Property and Casualty Insurance Products
Prior accident years reserve development
Prior accident year development [1]	(65,000,000)	(167,000,000)	(41,000,000)
Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit	(81,000,000)	(167,000,000)	(41,000,000)
PG&E [Member] | Portion of Expected Class Settlement Due to HIG [Member] | Minimum [Member]
Prior accident years reserve development
Gain Contingency, Unrecorded Amount	300,000,000
PG&E [Member] | Portion of Expected Class Settlement Due to HIG [Member] | Maximum
Prior accident years reserve development
Gain Contingency, Unrecorded Amount	325,000,000
Property and Casualty, Commercial Insurance [Member] | Property
Prior accident years reserve development
Prior accident year development [1]	(11,000,000)	(12,000,000)	(8,000,000)
Property and Casualty, Commercial Insurance [Member] | Automobile liability
Prior accident years reserve development
Prior accident year development [1]	27,000,000	(15,000,000)	17,000,000
Property and Casualty, Personal Insurance [Member] | Automobile liability
Prior accident years reserve development
Prior accident year development [1]	$ (38,000,000)	$ (18,000,000)	$ 0