Schedule VI (SUPPLEMENTARY INFORMATION CONCERNING PROPERTY AND CASUALTY OPERATIONS) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred Policy Acquisition Costs		$ 4,244	$ 4,161	$ 3,926
Net Reserves for Unpaid Losses		53,164	48,509	48,271
Unearned Premiums		17,652	16,771	15,532
Net Premiums Earned		31,800	30,189	29,042
Net Investment Income		3,074	3,141	3,047
Current Year Claim and Claim Adjustment Expense		22,124	19,575	19,048
Prior Year Claim and Claim Adjustment Expense		(414)	(845)	(981)
Net Losses and Loss Expenses Incurred Related to Prior Year	[1]	(414)	(845)	(981)
Amortization of Deferred Policy Acquisition Costs		6,076	5,831	5,630
Net Paid Losses and Loss Expenses		17,434	18,473	18,340
Net Premiums Written		$ 32,471	$ 31,126	$ 29,505

[1] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $19 million, $53 million, and $85 million for 2020, 2019, and 2018, respectively.