Reinsurance - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended			60 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2021
Reinsurance, Loss on Uncollectible Accounts in Period, Amount	$ (1)	$ 0
Asbestos and Environmental
Policyholder Benefits and Claims Incurred, Ceded		220		$ 1,015
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	485
Adverse Development Cover Navigators Group [Member]
Policyholder Benefits and Claims Incurred, Ceded	300
Adverse Development Cover Navigators Group [Member] | Remaining [Member]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	0
Asbestos and Environmental
Policyholder Benefits and Claims Incurred, Ceded		220
Asbestos and Environmental | Asbestos and Environmental
Policyholder Benefits and Claims Incurred, Ceded	1,015
Asbestos and Environmental | Remaining [Member] | Asbestos and Environmental
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	485
The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member]
Policyholder Benefits and Claims Incurred, Ceded			$ 91
Maximum | Asbestos and Environmental
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	1,500
Maximum | Adverse Development Cover Navigators Group [Member]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	300
Maximum | Asbestos and Environmental | Asbestos and Environmental
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	1,500
Maximum | The Navigators Group, Inc. [Member] | Adverse Development Cover Navigators Group [Member]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			300
Property and Casualty Insurance Products
Policyholder Benefits and Claims Incurred, Ceded	1,243	1,156	826
Group Insurance Policy [Member]
Policyholder Benefits and Claims Incurred, Ceded	$ 85	$ 63	$ 73