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, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	$ 18,701	$ 17,885	$ 17,965	$ 18,070
Property and Casualty Operations [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	14,924	$ 14,153
Medical Professional Liability [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	1,520
Other Professional Liability and Management Liability [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	2,850
Surety [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	385
Commercial Auto [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	502
General Liability [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	3,305
Workers' Compensation [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	3,872
Property and Other [Member]
Net Liability for Unpaid Claim and Claim Adjustment Expenses [Abstract]
Net claim and claim adjustment expenses	$ 2,490