Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Net Liability for Unpaid Claims and Claims Adjustment Expenses by Line of Business (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	$ 19,205	$ 18,701	$ 17,885	$ 17,965
Property and casualty
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	15,270	$ 14,195
Medical Professional Liability
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	1,556
Other Professional Liability and Management Liability
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	3,159
Surety
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	406
Commercial Auto
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	673
General Liability
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	2,911
Workers' Compensation
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	3,850
Property and Other
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	$ 2,715