Insurance Claim Reserves (Details) - Prior Year Reserve Development - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Asbestos reserves [Member]
Liability for claims and claim adjustment expense
Increase to asbestos and environmental claims	$ 225	$ 225	$ 225
Environmental reserves [Member]
Liability for claims and claim adjustment expense
Increase to asbestos and environmental claims	55	65	82
Property-casualty [Member]
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	406	458	680
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	517	592	771
Property-casualty [Member] | Business Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	142	439	424
Property-casualty [Member] | Business Insurance [Member] | Asbestos reserves [Member]
Liability for claims and claim adjustment expense
Increase to asbestos and environmental claims	225	225	225
Property-casualty [Member] | Business Insurance [Member] | Environmental reserves [Member]
Liability for claims and claim adjustment expense
Increase to asbestos and environmental claims	55	65	82
Property-casualty [Member] | Bond & Specialty Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	266	140	350
Property-casualty [Member] | Personal Insurance [Member]
Liability for claims and claim adjustment expense
Net favorable (unfavorable) prior year reserve development impacting the Company's results of operations	$ 109	$ 13	$ (3)