Reserve for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Details) - USD ($)	3 Months Ended		7 Months Ended	12 Months Ended				36 Months Ended
	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Current accident year benefit from discounting property and casualty reserves				$ 33,000,000	$ 12,000,000	$ 15,000,000
Salvage and Subrogation Recoveries, Value	$ 57,000,000		$ 57,000,000	57,000,000				$ 57,000,000
Prepaid Reinsurance Premiums	57,000,000		57,000,000	57,000,000				57,000,000
Liability for Future Policy Benefit, Adverse Development, Expense	238,000,000					285,000,000
Reinsurance Recoverable, Guarantee Benefit					238,000,000	285,000,000
Change in Deferred Gain on Retroactive Reinsurance	16,000,000		16,000,000	16,000,000	0	0		16,000,000
Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Payments for Reinsurance		$ 91,000,000	91,000,000
Reinsurance Retention Policy, Amount Retained		68,000,000
Policyholder Benefits and Claims Incurred, Ceded		$ 91,000,000		107,000,000
Change in Deferred Gain on Retroactive Reinsurance	16,000,000		16,000,000	16,000,000				16,000,000
Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Amount Retained				100,000,000
Policyholder Benefits and Claims Incurred, Ceded				207,000,000
Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Amount Retained				1,816,000,000
Property and Casualty Insurance Products
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	22,986,000,000		22,986,000,000	22,986,000,000	20,352,000,000	19,818,000,000	$ 19,057,000,000	22,986,000,000
Reinsurance and Other Recoverables	5,275,000,000		5,275,000,000	5,275,000,000	4,232,000,000	3,957,000,000	3,488,000,000	5,275,000,000
Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	8,009,000,000		8,009,000,000	8,009,000,000	8,206,000,000	8,303,000,000	5,564,000,000	8,009,000,000
Prior accident year development [1]				(410,000,000)	(324,000,000)	(185,000,000)
Reinsurance and Other Recoverables	247,000,000		247,000,000	$ 247,000,000	239,000,000	$ 209,000,000	208,000,000	247,000,000
Scenario, Adjustment [Member] | Property and Casualty Insurance Products
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance and Other Recoverables					$ 90,000,000
Minimum | Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Range of discount rates				2.10%	2.10%	2.10%
Maximum | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured				$ 300,000,000
Maximum | Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Range of discount rates				8.00%	8.00%	8.00%
Camp Wildfire [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments	12,000,000		12,000,000	$ 12,000,000				12,000,000
Policyholder Benefits and Claims Incurred, Assumed				350,000,000
Group long-term disability | Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	6,616,000,000		6,616,000,000	6,616,000,000				6,616,000,000
Prior accident year development [1]				340,000,000	$ 230,000,000	$ 125,000,000
Reinsurance and Other Recoverables	236,000,000		236,000,000	236,000,000				236,000,000
Group life and accident, excluding premium waiver | Group Benefits
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	580,000,000		580,000,000	580,000,000				580,000,000
Prior accident year development [1]				60,000,000	90,000,000	60,000,000
Reinsurance and Other Recoverables	1,000,000		1,000,000	1,000,000				1,000,000
Property and Casualty Insurance Products
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	22,986,000,000		22,986,000,000	22,986,000,000				22,986,000,000
Prior accident year development [1]				(65,000,000)	(167,000,000)	(41,000,000)
Reinsurance and Other Recoverables	5,275,000,000		5,275,000,000	5,275,000,000				5,275,000,000
Policyholder Benefits and Claims Incurred, Ceded				$ 826,000,000	$ 661,000,000	$ 901,000,000
Property and Casualty Insurance Products | Minimum
Liability for Claims and Claims Adjustment Expense [Line Items]
Range of discount rates				1.76%	1.77%	1.77%
Property and Casualty Insurance Products | Maximum
Liability for Claims and Claims Adjustment Expense [Line Items]
Range of discount rates				14.03%	14.15%	14.15%
Catastrophes
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	825,000,000		825,000,000	$ 825,000,000				825,000,000
Prior accident year development [1]				(42,000,000)	$ (49,000,000)	$ (16,000,000)
Catastrophes | Scenario, Adjustment [Member] | Property and Casualty Insurance Products
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net					850,000,000
Prior accident year development [1]					133,000,000
Catastrophes | Scenario, Plan [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments	45,000,000		45,000,000	45,000,000				45,000,000
Asbestos and Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,147,000,000		1,147,000,000	1,147,000,000				1,147,000,000
Reinsurance and Other Recoverables	1,219,000,000		1,219,000,000	1,219,000,000				1,219,000,000
Payments for Reinsurance						650,000,000
Reinsurance Retention Policy, Excess Retention, Amount Reinsured				860,000,000
Reinsurance Retention Policy, Amount Retained							$ 1,700,000,000
Policyholder Benefits and Claims Incurred, Ceded				640,000,000				640,000,000
Asbestos and Environmental | Maximum
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured				1,500,000,000
General liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,494,000,000		3,494,000,000	3,494,000,000				3,494,000,000
Prior accident year development [1]				61,000,000	52,000,000	11,000,000
Reinsurance and Other Recoverables	630,000,000		630,000,000	630,000,000				630,000,000
General liability | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				28,000,000
Professional liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,050,000,000		1,050,000,000	1,050,000,000				1,050,000,000
Prior accident year development [1]				29,000,000	(12,000,000)	1,000,000
Reinsurance and Other Recoverables	549,000,000		549,000,000	549,000,000				549,000,000
Professional liability | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				25,000,000
Assumed Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	190,000,000		190,000,000	190,000,000				190,000,000
Prior accident year development [1]				3,000,000	0	0
Reinsurance and Other Recoverables	25,000,000		25,000,000	25,000,000				25,000,000
Assumed Reinsurance [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				13,000,000
Commercial Auto Physical Damage [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	15,000,000		15,000,000	15,000,000				15,000,000
Reinsurance and Other Recoverables	(1,000,000)		(1,000,000)	(1,000,000)				(1,000,000)
Commercial Auto Physical Damage [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				12,000,000
Marine [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	279,000,000		279,000,000	279,000,000				279,000,000
Prior accident year development [1]				8,000,000	0	0
Reinsurance and Other Recoverables	135,000,000		135,000,000	135,000,000				135,000,000
Marine [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				9,000,000
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior accident year development [1]				3,000,000	(25,000,000)	(14,000,000)
Property | Property and Casualty, Commercial Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	461,000,000		461,000,000	461,000,000				461,000,000
Prior accident year development [1]				(11,000,000)	$ (12,000,000)	$ (8,000,000)
Reinsurance and Other Recoverables	162,000,000		162,000,000	162,000,000				162,000,000
Property | Property and Casualty, Commercial Insurance [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				8,000,000
Short-Duration Insurance, Other [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	215,000,000		215,000,000	215,000,000				215,000,000
Reinsurance and Other Recoverables	312,000,000		312,000,000	312,000,000				312,000,000
Short-Duration Insurance, Other [Member] | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Ceded				12,000,000
PG&E [Member] | Portion of Expected Class Settlement Due to HIG [Member] | Minimum
Gain Contingencies [Line Items]
Gain Contingency, Unrecorded Amount	300,000,000		300,000,000	300,000,000				300,000,000
PG&E [Member] | Portion of Expected Class Settlement Due to HIG [Member] | Maximum
Gain Contingencies [Line Items]
Gain Contingency, Unrecorded Amount	$ 325,000,000		$ 325,000,000	$ 325,000,000				$ 325,000,000