Insurance Claim Reserves (Details) - Prior Year Reserve Development - Property-casualty [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for claims and claim adjustment expense
Net favorable prior year reserve development included in estimated claims and claim adjustment expenses incurred for claims arising in prior years	$ 294	$ 3
Net favorable prior year reserve development impacting the Company's results of operations	317	27
Accretion expense	12	12
Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	134	5
Subrogation benefit	10
Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	15	0
Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable prior year reserve development impacting the Company's results of operations	168	$ 22
Subrogation benefit	$ 62